fix(app): add modules.txt/patches.txt so bench keeps ns_app in apps.txt

ns_app was hand-created without the standard scaffolding files, so it was
missing modules.txt and patches.txt. bench's is_frappe_app() check requires
all of hooks.py, modules.txt, and patches.txt to exist; BenchApps.sync()
rebuilds sites/apps.txt from the folders that pass that check. sync() runs on
bench get-app, install-app (of any app), update, etc., so every such command
silently dropped ns_app from apps.txt.

Once ns_app was missing from apps.txt, `bench build --app ns_app` crashed
(esbuild get_public_path returned undefined), the sites/assets/ns_app symlink
was never created, and /assets/ns_app/js/customer_statements.js 404'd. That
left ns_statements undefined, so the Customer "Generate Statements" button's
handler did nothing and the statement screen never opened on production.

Add modules.txt ("NS App", matching app_title), an empty patches.txt, and the
"NS App" module package folder so bench recognizes ns_app as a Frappe app and
stops removing it. Run `bench --site <site> migrate` to register the module.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/CUSTOMER_STATEMENTS.md

@@ -0,0 +1,135 @@
+# Customer Statements & Late Payment Fees
+
+> Branch: `feature/customer-statements`
+
+Generates printable, **one-page-per-customer** account statements — formatted to
+fit a standard #9 (9x4) **double-window envelope** — for customers with overdue
+invoices, and (optionally) bills a **late-payment fee** that posts to the ledger
+and is collectible through the app's existing payment flow.
+
+---
+
+## 1. What it does
+
+From either the **Customer list** or a **Customer form**, a user can generate
+account statements:
+
+1. **Pick customers.** On the list, *Generate Statements* opens a dialog listing
+   every customer with overdue invoices (overdue count, max days overdue, total
+   outstanding) with select-all. On a Customer form, *Generate Statement* targets
+   that one customer.
+2. **Choose whether to bill a late fee** via a checkbox in the popup
+   (*Generate late payment fee*, on by default).
+3. **Get a printable report.** A new browser tab opens with one statement per
+   page — each showing the customer's open invoices, aging buckets
+   (Current / 1–30 / 31–60 / 61–90 / 90+), and a **Total Due**. The customer and
+   company (return) addresses sit in the two envelope-window positions.
+
+Each generation is recorded on the customer's timeline as an audit-trail entry.
+
+---
+
+## 2. Late payment fees
+
+### How the fee is calculated
+Interest uses ERPNext's own Dunning formula:
+
+```
+fee = Σ(invoice.outstanding × rate_of_interest/100/365 × days_overdue) + dunning_fee
+```
+
+### Where the settings live — ERPNext **Dunning Type**
+All fee configuration comes from the existing **Dunning Type** doctype
+(Accounting ▸ Dunning Type). Nothing is auto-created; generation stops with a
+clear error until it is configured. The default (`is_default`) Dunning Type for
+the company is used. Fields consumed:
+
+| Dunning Type field | Purpose |
+|--------------------|---------|
+| `rate_of_interest` | Annual interest rate (%) |
+| `dunning_fee` | Flat fee added per statement |
+| `income_account` | Credited when the fee is billed |
+| `cost_center` | Cost center for the fee line (falls back to company default) |
+| `custom_late_fee_item` | **Late Fee Item** — the Item used to bill the fee (custom field added by this app) |
+
+### How the fee posts, and how it gets paid
+The fee is billed as a **submitted Sales Invoice** (item → the Dunning Type
+income account). This is deliberate: because it is a real Sales Invoice it
+
+- increases the customer's receivable balance immediately, and
+- appears in `get_unpaid_invoices` and is charged/settled automatically by the
+  app's existing payment paths (**Run Payment / AutoPay / multi-invoice** →
+  `create_payment_entry`), which allocate against Sales Invoices.
+
+A bare Journal Entry (or an ERPNext Dunning document) would raise the balance but
+sit **uncollectible** by those flows — hence the Sales Invoice.
+
+### Fee invoice specifics
+- **Dedicated naming series `LPF-.YYYY.-`** (e.g. `LPF-2026-00001`) so late-fee
+  invoices are easy to spot and filter. Registered on Sales Invoice's
+  `naming_series` via `after_migrate`.
+- **Never taxed.** A single zero-amount "Actual" tax line keeps ERPNext from
+  auto-applying company/item tax templates, so the invoice total equals the
+  computed fee exactly and nothing extra hits the ledger.
+- **Idempotent** — at most one fee invoice per customer / company / calendar
+  month. Prior fee invoices are excluded from the interest base (no fee-on-fee).
+
+On the statement the fee shows as a normal invoice line tagged **“late fee”**,
+folded into a single **Total Due** that equals the customer's balance.
+
+---
+
+## 3. Configuration / prerequisites
+
+1. **Migrate** the app (`bench --site <site> migrate`) — creates the
+   `Late Fee Item` custom field on Dunning Type and registers the `LPF-` series.
+2. Create an **Item** to represent the fee (a non-stock sales item, e.g.
+   "Late Payment Fee").
+3. Create/complete a **Dunning Type** for the company with: rate of interest,
+   dunning fee, **income account**, and the **Late Fee Item**. Mark it default.
+
+If any of these is missing, statement generation throws a clear, actionable
+error and posts nothing.
+
+---
+
+## 4. Audit trail
+
+Each generated statement adds an *Info* comment to the customer's timeline, e.g.
+
+> Statement generated — Total Due $557.17 (late fee invoice LPF-2026-00001).
+
+The note reflects the outcome: the fee invoice raised, *no late fee*, or
+*late fee skipped* (when the fee checkbox was cleared). It is attributed to the
+generating user.
+
+---
+
+## 5. Files
+
+| File | Role |
+|------|------|
+| `ns_app/api/statements.py` | Overdue-customer query, statement builder, printable HTML, late-fee billing (Sales Invoice), audit-trail entry |
+| `ns_app/templates/statements/customer_statement.html` | Jinja template for one customer page (envelope windows + aging table) |
+| `ns_app/public/js/customer_statements.js` | List action + Customer-form button + selection/fee-toggle popups (loaded globally) |
+| `ns_app/setup.py` | `after_migrate`: creates the Late Fee Item custom field, registers the `LPF-` naming series |
+| `ns_app/hooks.py` | Wires the JS (`app_include_js`) and `after_migrate` |
+
+### Server API (`ns_app.api.statements`)
+- `get_customers_with_overdue_invoices()` — customers with overdue invoices.
+- `generate_statements(customers, skip_late_fee=0)` — bills fees (unless skipped),
+  renders the printable HTML, records the audit entry. Returns
+  `{html, rendered, skipped}`.
+- `get_statement_data(customer)` — the per-customer statement data (internal).
+
+Access is restricted to System Manager, Sales User/Manager, Accounts
+User/Manager.
+
+---
+
+## 6. Notes / non-goals
+
+- No persisted "Statement" doctype — statements are generated on demand.
+- No email/fax delivery — print only.
+- Whether the fee should be taxed and the interest rate/fee amounts are business
+  settings, controlled entirely through Dunning Type.

ns_app/api/statements.py

@@ -0,0 +1,498 @@
+"""Customer account statements.
+
+Generates printable, one-customer-per-page account statements for customers with
+overdue invoices, formatted for a standard double-window envelope. Statement
+generation also books a late-payment fee to the ledger (see the late-fee helpers
+added alongside the generator).
+"""
+
+import json
+
+import frappe
+from frappe import _
+from frappe.contacts.doctype.address.address import get_address_display, get_default_address
+from frappe.utils import flt, fmt_money, getdate, nowdate
+
+# Dedicated naming series so late-fee invoices are easy to spot and filter.
+LATE_FEE_NAMING_SERIES = "LPF-.YYYY.-"
+
+# Roles allowed to run collections/statement actions.
+ALLOWED_ROLES = [
+    "System Manager",
+    "Sales User",
+    "Sales Manager",
+    "Accounts User",
+    "Accounts Manager",
+]
+
+
+@frappe.whitelist()
+def get_customers_with_overdue_invoices():
+    """Return one row per customer that has at least one overdue Sales Invoice.
+
+    A Sales Invoice is overdue when it is submitted, still has an outstanding
+    balance, and its due date is in the past.
+    """
+    frappe.only_for(ALLOWED_ROLES)
+
+    today = nowdate()
+    rows = frappe.get_all(
+        "Sales Invoice",
+        filters={
+            "docstatus": 1,
+            "outstanding_amount": [">", 0],
+            "due_date": ["<", today],
+        },
+        fields=[
+            "customer",
+            "customer_name",
+            "count(name) as overdue_count",
+            "sum(outstanding_amount) as total_outstanding",
+            "min(due_date) as oldest_due_date",
+        ],
+        group_by="customer, customer_name",
+        order_by="total_outstanding desc",
+    )
+
+    for row in rows:
+        row["max_days_overdue"] = (
+            (getdate(today) - getdate(row.oldest_due_date)).days
+            if row.oldest_due_date
+            else 0
+        )
+
+    return rows
+
+
+def _aging_bucket(days_overdue):
+    """Map days-overdue to a standard aging bucket label."""
+    if days_overdue <= 0:
+        return "Current"
+    if days_overdue <= 30:
+        return "1-30"
+    if days_overdue <= 60:
+        return "31-60"
+    if days_overdue <= 90:
+        return "61-90"
+    return "90+"
+
+
+def _get_outstanding_invoices(customer):
+    """Return all open (submitted, unpaid) Sales Invoices for a customer.
+
+    The statement lists the full open balance, so this includes not-yet-due
+    invoices; each row is annotated with days overdue, an overdue flag, and its
+    aging bucket.
+    """
+    today = getdate(nowdate())
+    invoices = frappe.get_all(
+        "Sales Invoice",
+        filters={
+            "customer": customer,
+            "docstatus": 1,
+            "outstanding_amount": [">", 0],
+        },
+        fields=[
+            "name",
+            "posting_date",
+            "due_date",
+            "outstanding_amount",
+            "grand_total",
+            "company",
+        ],
+        order_by="due_date asc",
+    )
+
+    for inv in invoices:
+        due = getdate(inv.due_date) if inv.due_date else None
+        days = (today - due).days if due else 0
+        inv["days_overdue"] = days if days > 0 else 0
+        inv["is_overdue"] = days > 0
+        inv["aging_bucket"] = _aging_bucket(inv["days_overdue"])
+
+    return invoices
+
+
+def _address_display(doctype, name):
+    """Return the formatted (HTML) default address for a party, or ''."""
+    address_name = get_default_address(doctype, name)
+    if not address_name:
+        return ""
+    return get_address_display(frappe.get_doc("Address", address_name).as_dict()) or ""
+
+
+def _resolve_company(invoices):
+    """Pick the company for the statement header/return address."""
+    if invoices:
+        return invoices[0].company
+    return frappe.defaults.get_user_default("Company") or frappe.db.get_single_value(
+        "Global Defaults", "default_company"
+    )
+
+
+def get_statement_data(customer, invoices=None):
+    """Assemble everything the statement template needs for one customer.
+
+    Late-fee charges are billed as Sales Invoices, so they appear in the invoice
+    list like any other open item (flagged `is_late_fee`); there is no separate
+    fee total to add.
+    """
+    cust = frappe.get_doc("Customer", customer)
+    if invoices is None:
+        invoices = _get_outstanding_invoices(customer)
+
+    fee_names = _late_fee_invoice_names(customer)
+
+    company = _resolve_company(invoices)
+    company_doc = frappe.get_doc("Company", company) if company else None
+
+    aging = {"Current": 0.0, "1-30": 0.0, "31-60": 0.0, "61-90": 0.0, "90+": 0.0}
+    total_due = 0.0
+    for inv in invoices:
+        inv["is_late_fee"] = inv["name"] in fee_names
+        aging[inv["aging_bucket"]] += flt(inv["outstanding_amount"])
+        total_due += flt(inv["outstanding_amount"])
+
+    return {
+        "customer": cust.name,
+        "customer_name": cust.customer_name,
+        "customer_address": _address_display("Customer", cust.name),
+        "company": company,
+        "company_name": company_doc.company_name if company_doc else "",
+        "return_address": _address_display("Company", company) if company else "",
+        "currency": (company_doc.default_currency if company_doc else None)
+        or frappe.db.get_single_value("Global Defaults", "default_currency"),
+        "invoices": invoices,
+        "aging": aging,
+        "total_due": total_due,
+        "statement_date": nowdate(),
+    }
+
+
+def _render_page(data):
+    path = frappe.get_app_path(
+        "ns_app", "templates", "statements", "customer_statement.html"
+    )
+    with open(path) as f:
+        template = f.read()
+    return frappe.render_template(template, {"s": data})
+
+
+def _wrap_document(pages):
+    """Wrap rendered per-customer pages in a printable HTML document."""
+    body = "\n".join(pages)
+    return f"""<!DOCTYPE html>
+<html>
+<head>
+<meta charset="utf-8">
+<title>Customer Statements</title>
+<style>
+  @page {{ size: Letter; margin: 0; }}
+  * {{ box-sizing: border-box; }}
+  body {{ margin: 0; font-family: Helvetica, Arial, sans-serif; color: #333; }}
+  .toolbar {{ text-align: center; padding: 12px; background: #f5f5f5; }}
+  .toolbar button {{ font-size: 14px; padding: 8px 20px; cursor: pointer; }}
+  .statement-page {{
+    position: relative;
+    width: 8.5in;
+    min-height: 11in;
+    margin: 0 auto;
+    padding: 0;
+    page-break-after: always;
+    overflow: hidden;
+  }}
+  .statement-page:last-of-type {{ page-break-after: auto; }}
+  /* Window positions are field-tuned to the #9 (9x4) double-window envelope
+     (verified against a printed proof). */
+  .return-window {{
+    position: absolute; top: 0.8in; left: 0.6in;
+    width: 3.5in; font-size: 11px; line-height: 1.3;
+  }}
+  .doc-header {{
+    position: absolute; top: 0.8in; right: 0.6in;
+    width: 3in; text-align: right; font-size: 13px; line-height: 1.5;
+  }}
+  .doc-header .doc-title {{ font-size: 20px; font-weight: bold; letter-spacing: 1px; }}
+  .recipient-window {{
+    position: absolute; top: 2.5in; left: 1.125in;
+    width: 4.5in; height: 1.25in; font-size: 15px; line-height: 1.15em;
+    overflow: hidden;
+  }}
+  .statement-body {{ padding: 3.9in 0.6in 0.6in 0.6in; }}
+  .intro {{ font-size: 12px; margin-bottom: 12px; }}
+  table.items, table.aging {{ width: 100%; border-collapse: collapse; }}
+  table.items th, table.items td,
+  table.aging th, table.aging td {{
+    border: 1px solid #ccc; padding: 6px; font-size: 13px;
+  }}
+  table.items th, table.aging th {{ background: #f5f5f5; text-align: left; }}
+  .c {{ text-align: center; }}
+  .r {{ text-align: right; }}
+  tr.overdue td {{ color: #c62828; }}
+  .tag {{
+    display: inline-block; font-size: 10px; font-weight: bold; color: #fff;
+    background: #c62828; border-radius: 3px; padding: 1px 5px; vertical-align: middle;
+  }}
+  .totals {{ width: 45%; margin: 12px 0 12px auto; font-size: 14px; }}
+  .totals p {{ display: flex; justify-content: space-between; margin: 4px 0; }}
+  .totals p.grand {{
+    border-top: 2px solid #333; padding-top: 6px; font-weight: bold; font-size: 16px;
+  }}
+  table.aging {{ margin-top: 8px; }}
+  .footer {{
+    margin-top: 24px; font-size: 10px; color: #777; text-align: center;
+    white-space: pre-line;
+  }}
+  @media print {{ .toolbar {{ display: none; }} }}
+</style>
+</head>
+<body>
+  <div class="toolbar">
+    <button onclick="window.print()">Print Statements</button>
+  </div>
+  {body}
+</body>
+</html>"""
+
+
+# ── Late-payment fee (billed as a Sales Invoice on generation) ───────────────
+#
+# Fee schedule/amounts come from ERPNext's existing **Dunning Type** settings
+# (rate_of_interest is a yearly %, plus a flat dunning_fee), editable in the
+# desk. Interest is computed with ERPNext's own Dunning formula so the numbers
+# match a Dunning document. The fee is billed as a submitted **Sales Invoice**
+# (item -> Dunning Type income account) so it both hits the ledger and is
+# collectible by the app's existing payment flow (Run Payment / AutoPay /
+# multi-invoice), which settles Sales Invoices.
+
+DUNNING_TYPE_FIELDS = [
+    "name",
+    "rate_of_interest",
+    "dunning_fee",
+    "income_account",
+    "cost_center",
+    "company",
+    "custom_late_fee_item",
+]
+
+
+def _get_fee_settings(company):
+    """Resolve the Dunning Type used for late fees for a company.
+
+    Nothing is auto-created: the user must configure a Dunning Type (rate of
+    interest, fee, income account) for the company in ERPNext. If none exists we
+    stop with a clear, actionable error rather than inventing default values.
+    """
+
+    def first(filters):
+        rows = frappe.get_all(
+            "Dunning Type", filters=filters, fields=DUNNING_TYPE_FIELDS, limit=1
+        )
+        return rows[0] if rows else None
+
+    dt = first({"company": company, "is_default": 1}) or first({"company": company})
+    if not dt:
+        frappe.throw(
+            _(
+                "No Dunning Type is configured for {0}. Create one under "
+                "Accounting > Dunning Type — set the rate of interest, dunning "
+                "fee, and income account — before generating statements."
+            ).format(company)
+        )
+    return dt
+
+
+def _late_fee_period():
+    """Statement period key used for idempotency (one fee per calendar month)."""
+    return getdate(nowdate()).strftime("%Y-%m")
+
+
+def _get_fee_invoices(customer, company, fee_item):
+    """Return submitted late-fee Sales Invoices for a customer (by fee item)."""
+    if not fee_item:
+        return []
+    return frappe.db.sql(
+        """
+        select si.name, si.posting_date
+        from `tabSales Invoice` si
+        inner join `tabSales Invoice Item` sii on sii.parent = si.name
+        where si.customer = %s and si.company = %s and si.docstatus = 1
+          and sii.item_code = %s
+        """,
+        (customer, company, fee_item),
+        as_dict=True,
+    )
+
+
+def _post_late_fee_invoice(customer, company, overdue_invoices, period):
+    """Bill a late fee as a submitted Sales Invoice (idempotent per month).
+
+    Returns the fee invoice name, or None if nothing was billed.
+    """
+    if not overdue_invoices:
+        return None
+
+    settings = _get_fee_settings(company)
+    fee_item = settings.get("custom_late_fee_item")
+    if not fee_item:
+        frappe.throw(
+            _("Set a Late Fee Item on Dunning Type {0} before generating statements.").format(
+                settings.name
+            )
+        )
+    if not settings.income_account:
+        frappe.throw(
+            _("Set an Income Account on Dunning Type {0} before generating statements.").format(
+                settings.name
+            )
+        )
+
+    fee_invoices = _get_fee_invoices(customer, company, fee_item)
+
+    # Idempotency: at most one fee invoice per (customer, company, month).
+    month_start = getdate(period + "-01")
+    for fi in fee_invoices:
+        if getdate(fi.posting_date) >= month_start:
+            return fi.name
+
+    # Interest on overdue balances, excluding prior fee invoices (no fee-on-fee).
+    prior_fee_names = {fi.name for fi in fee_invoices}
+    daily_interest = flt(settings.rate_of_interest) / 100.0 / 365.0
+    interest = sum(
+        flt(inv["outstanding_amount"]) * daily_interest * inv["days_overdue"]
+        for inv in overdue_invoices
+        if inv["name"] not in prior_fee_names
+    )
+    fee = round(interest + flt(settings.dunning_fee), 2)
+    if fee <= 0:
+        return None
+
+    cost_center = settings.cost_center or frappe.get_cached_value(
+        "Company", company, "cost_center"
+    )
+
+    si = frappe.new_doc("Sales Invoice")
+    si.naming_series = LATE_FEE_NAMING_SERIES
+    si.customer = customer
+    si.company = company
+    si.posting_date = nowdate()
+    si.due_date = nowdate()
+    si.append(
+        "items",
+        {
+            "item_code": fee_item,
+            "qty": 1,
+            "rate": fee,
+            "income_account": settings.income_account,
+            "cost_center": cost_center,
+            "description": _("Late payment fee for statement period {0}").format(period),
+        },
+    )
+    # Late fees are not taxed. A single zero "Actual" tax line keeps the taxes
+    # table non-empty, which stops ERPNext from auto-applying the company or
+    # item tax templates; being zero it posts nothing to the ledger.
+    si.taxes_and_charges = ""
+    si.append(
+        "taxes",
+        {
+            "charge_type": "Actual",
+            "account_head": settings.income_account,
+            "description": _("Late fees are not taxed"),
+            "tax_amount": 0,
+            "rate": 0,
+        },
+    )
+    si.insert(ignore_permissions=True)
+    si.submit()
+    frappe.db.commit()
+    return si.name
+
+
+def _late_fee_invoice_names(customer):
+    """Names of the customer's submitted late-fee Sales Invoices (any company)."""
+    fee_items = [
+        d.custom_late_fee_item
+        for d in frappe.get_all("Dunning Type", fields=["custom_late_fee_item"])
+        if d.custom_late_fee_item
+    ]
+    if not fee_items:
+        return set()
+    rows = frappe.db.sql(
+        """
+        select distinct sii.parent
+        from `tabSales Invoice Item` sii
+        inner join `tabSales Invoice` si on si.name = sii.parent
+        where si.customer = %s and si.docstatus = 1 and sii.item_code in %s
+        """,
+        (customer, tuple(fee_items)),
+        as_dict=True,
+    )
+    return {r.parent for r in rows}
+
+
+def _record_statement_activity(customer, data, fee_invoice_names, skip_late_fee):
+    """Log statement generation on the customer's timeline (audit trail)."""
+    total = fmt_money(data["total_due"], currency=data["currency"])
+    if skip_late_fee:
+        fee_note = _("late fee skipped")
+    elif fee_invoice_names:
+        fee_note = _("late fee invoice {0}").format(", ".join(fee_invoice_names))
+    else:
+        fee_note = _("no late fee")
+    frappe.get_doc("Customer", customer).add_comment(
+        "Info", _("Statement generated — Total Due {0} ({1}).").format(total, fee_note)
+    )
+
+
+@frappe.whitelist()
+def generate_statements(customers, skip_late_fee=0):
+    """Render printable statements (one page per customer) for the selection.
+
+    Side effect (unless `skip_late_fee`): a late-payment fee is billed as a
+    Sales Invoice (once per customer per month) for each customer with overdue
+    invoices. Each generation is recorded on the customer's timeline.
+
+    `customers` may arrive as a JSON-encoded list from the client.
+    """
+    frappe.only_for(ALLOWED_ROLES)
+
+    if isinstance(customers, str):
+        try:
+            customers = json.loads(customers)
+        except (ValueError, TypeError):
+            customers = [customers]
+    if not customers:
+        frappe.throw(_("No customers selected"))
+    skip_late_fee = int(skip_late_fee or 0)
+
+    period = _late_fee_period()
+    pages, rendered, skipped = [], [], []
+
+    for customer in customers:
+        invoices = _get_outstanding_invoices(customer)
+        if not invoices:
+            skipped.append(customer)
+            continue
+
+        # Bill the late fee per company (on overdue invoices only).
+        fee_invoice_names = []
+        if not skip_late_fee:
+            overdue_by_company = {}
+            for inv in invoices:
+                if inv["is_overdue"]:
+                    overdue_by_company.setdefault(inv["company"], []).append(inv)
+            for comp, invs in overdue_by_company.items():
+                name = _post_late_fee_invoice(customer, comp, invs, period)
+                if name:
+                    fee_invoice_names.append(name)
+
+        # Re-fetch so the statement includes the freshly billed fee invoice(s).
+        data = get_statement_data(customer)
+        pages.append(_render_page(data))
+        rendered.append(customer)
+        _record_statement_activity(customer, data, fee_invoice_names, skip_late_fee)
+
+    if not pages:
+        frappe.throw(_("None of the selected customers have an outstanding balance."))
+
+    return {"html": _wrap_document(pages), "rendered": rendered, "skipped": skipped}

ns_app/hooks.py

@@ -7,7 +7,8 @@ app_license = "MIT"
 
 # Load on every page
 app_include_js = [
-    "/assets/ns_app/js/customer_quick_entry.js"
+    "/assets/ns_app/js/customer_quick_entry.js",
+    "/assets/ns_app/js/customer_statements.js"
 ]
 
 # Load on Sales Invoice form
@@ -15,6 +16,14 @@ doctype_js = {
     "Sales Invoice": "public/js/sales_invoice.js"
 }
 
+# Load on Customer list view (merges the "Generate Statements" action)
+doctype_list_js = {
+    "Customer": "public/js/customer_list.js"
+}
+
+# Ensure custom fields exist after every migrate
+after_migrate = "ns_app.setup.after_migrate"
+
 # Fixtures tracked in Git
 fixtures = [
     {

ns_app/modules.txt

@@ -0,0 +1 @@
+NS App

ns_app/public/js/customer_list.js

@@ -0,0 +1,20 @@
+// Customer list action: "Generate Statements". Registered as a doctype_list_js
+// so it loads alongside ERPNext's own Customer list settings (in app order,
+// after them). We MERGE into listview_settings — preserving any existing
+// onload / add_fields — instead of reassigning the object, which would clobber
+// ERPNext's settings (and be clobbered by them). The shared generate/print
+// helpers live on `ns_statements` (public/js/customer_statements.js).
+
+frappe.listview_settings["Customer"] = frappe.listview_settings["Customer"] || {};
+
+(function () {
+    const settings = frappe.listview_settings["Customer"];
+    const original_onload = settings.onload;
+
+    settings.onload = function (listview) {
+        if (original_onload) original_onload(listview);
+        listview.page.add_inner_button(__("Generate Statements"), () => {
+            ns_statements.pick_and_generate();
+        });
+    };
+})();

ns_app/public/js/customer_statements.js

@@ -0,0 +1,200 @@
+// Customer Statements: generate printable, one-page-per-customer account
+// statements formatted for a window envelope. Two entry points share the same
+// generate/print helpers — a multi-select action on the Customer list and a
+// single-customer button on the Customer form. Loaded globally so both the
+// list view and the form can reach the shared `ns_statements` helpers.
+
+frappe.provide("ns_statements");
+
+// The Customer list button is registered separately in customer_list.js
+// (a doctype_list_js) so it merges with — rather than overwrites — ERPNext's
+// own listview_settings["Customer"]. Shared helpers live here on ns_statements.
+
+// ── Entry point: Customer form ───────────────────────────────────────────────
+frappe.ui.form.on("Customer", {
+    refresh(frm) {
+        if (frm.is_new()) return;
+        frm.add_custom_button(__("Generate Statement"), () => {
+            ns_statements.generate_for_customer(frm.doc.name);
+        });
+    }
+});
+
+// ── Shared: call the backend and open the printable document ─────────────────
+ns_statements.run = function (customers, skip_late_fee) {
+    frappe.call({
+        method: "ns_app.api.statements.generate_statements",
+        args: { customers, skip_late_fee: skip_late_fee ? 1 : 0 },
+        freeze: true,
+        freeze_message: __("Generating statements..."),
+        callback(r) {
+            if (!r.message || !r.message.html) return;
+            ns_statements.open_print_window(r.message.html);
+            const skipped = (r.message.skipped || []).length;
+            if (skipped) {
+                frappe.show_alert({
+                    message: __("Skipped {0} customer(s) with no balance.", [skipped]),
+                    indicator: "orange"
+                });
+            }
+        }
+    });
+};
+
+ns_statements.open_print_window = function (html) {
+    const w = window.open("", "_blank");
+    if (!w) {
+        frappe.msgprint(__("Please allow pop-ups to view the statements."));
+        return;
+    }
+    w.document.open();
+    w.document.write(html);
+    w.document.close();
+};
+
+// ── List flow: pick customers with overdue invoices, then generate ───────────
+ns_statements.pick_and_generate = function () {
+    frappe.call({
+        method: "ns_app.api.statements.get_customers_with_overdue_invoices",
+        freeze: true,
+        freeze_message: __("Finding customers with overdue invoices..."),
+        callback(r) {
+            const rows = r.message || [];
+            if (!rows.length) {
+                frappe.msgprint({
+                    title: __("No Overdue Customers"),
+                    message: __("No customers currently have overdue invoices."),
+                    indicator: "green"
+                });
+                return;
+            }
+            ns_statements._selection_dialog(rows);
+        }
+    });
+};
+
+ns_statements._selection_dialog = function (rows) {
+    const uid = Date.now();
+    const selected = new Set(rows.map(r => r.customer)); // default: all selected
+
+    const body = rows.map(r => `
+        <tr>
+          <td class="text-center">
+            <input type="checkbox" class="cust-check-${uid}"
+                   data-name="${frappe.utils.escape_html(r.customer)}" checked>
+          </td>
+          <td>${frappe.utils.escape_html(r.customer_name || r.customer)}</td>
+          <td class="text-center">${r.overdue_count}</td>
+          <td class="text-center">${r.max_days_overdue}</td>
+          <td class="text-right">${format_currency(r.total_outstanding)}</td>
+        </tr>`).join("");
+
+    const dialog = new frappe.ui.Dialog({
+        title: __("Generate Customer Statements"),
+        size: "large",
+        fields: [
+            {
+                fieldtype: "HTML",
+                fieldname: "selector",
+                options: `
+                    <div style="max-height:45vh; overflow:auto;">
+                      <table class="table table-bordered table-sm" style="font-size:13px; margin:0;">
+                        <thead style="position:sticky; top:0; background:#f5f5f5;">
+                          <tr>
+                            <th style="width:36px;">
+                              <input type="checkbox" id="sel_all_${uid}" title="${__("Select all")}" checked>
+                            </th>
+                            <th>${__("Customer")}</th>
+                            <th class="text-center">${__("Overdue Invoices")}</th>
+                            <th class="text-center">${__("Max Days Overdue")}</th>
+                            <th class="text-right">${__("Total Outstanding")}</th>
+                          </tr>
+                        </thead>
+                        <tbody id="sel_body_${uid}">${body}</tbody>
+                      </table>
+                    </div>
+                    <div id="sel_count_${uid}" style="margin-top:8px; font-weight:bold;"></div>`
+            },
+            {
+                fieldtype: "Check",
+                fieldname: "generate_late_fee",
+                label: __("Generate late payment fee"),
+                default: 1,
+                description: __("Bills a late-fee invoice (once per customer this month) for overdue balances.")
+            }
+        ],
+        primary_action_label: __("Generate Statements"),
+        primary_action() {
+            const customers = [...selected];
+            if (!customers.length) {
+                frappe.msgprint(__("Select at least one customer."));
+                return;
+            }
+            const gen_fee = dialog.get_value("generate_late_fee");
+            const proceed = () => {
+                dialog.hide();
+                ns_statements.run(customers, !gen_fee);
+            };
+            if (gen_fee) {
+                frappe.confirm(
+                    __("Generate statements for {0} customer(s)? A late-fee invoice will be raised (once per customer this month) for any overdue balances.", [customers.length]),
+                    proceed
+                );
+            } else {
+                proceed();
+            }
+        }
+    });
+
+    dialog.show();
+
+    const update_count = () => {
+        const el = document.getElementById(`sel_count_${uid}`);
+        if (el) el.innerText = __("{0} of {1} selected", [selected.size, rows.length]);
+    };
+    update_count();
+
+    dialog.$wrapper.on("change", `.cust-check-${uid}`, function () {
+        if (this.checked) selected.add(this.dataset.name);
+        else selected.delete(this.dataset.name);
+        const all = dialog.$wrapper[0].querySelectorAll(`.cust-check-${uid}`);
+        const selAll = document.getElementById(`sel_all_${uid}`);
+        if (selAll) selAll.checked = [...all].every(c => c.checked);
+        update_count();
+    });
+
+    dialog.$wrapper.on("change", `#sel_all_${uid}`, function () {
+        dialog.$wrapper[0].querySelectorAll(`.cust-check-${uid}`).forEach(cb => {
+            cb.checked = this.checked;
+            if (this.checked) selected.add(cb.dataset.name);
+            else selected.delete(cb.dataset.name);
+        });
+        update_count();
+    });
+};
+
+// ── Form flow: single customer, with the same fee toggle ─────────────────────
+ns_statements.generate_for_customer = function (customer) {
+    const d = new frappe.ui.Dialog({
+        title: __("Generate Statement"),
+        fields: [
+            {
+                fieldtype: "HTML",
+                options: `<p>${__("Generate an account statement for <b>{0}</b>.", [frappe.utils.escape_html(customer)])}</p>`
+            },
+            {
+                fieldtype: "Check",
+                fieldname: "generate_late_fee",
+                label: __("Generate late payment fee"),
+                default: 1,
+                description: __("Bills a late-fee invoice (once this month) for overdue balances.")
+            }
+        ],
+        primary_action_label: __("Generate"),
+        primary_action(values) {
+            d.hide();
+            ns_statements.run([customer], !values.generate_late_fee);
+        }
+    });
+    d.show();
+};

ns_app/setup.py

@@ -0,0 +1,46 @@
+"""App setup: custom fields created/synced on migrate."""
+
+import frappe
+from frappe.custom.doctype.custom_field.custom_field import create_custom_fields
+from frappe.custom.doctype.property_setter.property_setter import make_property_setter
+
+from ns_app.api.statements import LATE_FEE_NAMING_SERIES
+
+# Fee schedule/amounts live on ERPNext's Dunning Type; this adds the one thing
+# it lacks — the Item used to bill a late fee as a Sales Invoice.
+CUSTOM_FIELDS = {
+    "Dunning Type": [
+        {
+            "fieldname": "custom_late_fee_item",
+            "label": "Late Fee Item",
+            "fieldtype": "Link",
+            "options": "Item",
+            "insert_after": "income_account",
+            "description": (
+                "Item used to bill a late-payment fee as a Sales Invoice when "
+                "customer statements are generated."
+            ),
+        }
+    ]
+}
+
+
+def _register_late_fee_naming_series():
+    """Add the late-fee series to Sales Invoice's naming_series options."""
+    field = frappe.get_meta("Sales Invoice").get_field("naming_series")
+    options = [o for o in (field.options or "").split("\n")] if field else []
+    if LATE_FEE_NAMING_SERIES not in options:
+        options.append(LATE_FEE_NAMING_SERIES)
+        make_property_setter(
+            "Sales Invoice",
+            "naming_series",
+            "options",
+            "\n".join(options),
+            "Text",
+            validate_fields_for_doctype=False,
+        )
+
+
+def after_migrate():
+    create_custom_fields(CUSTOM_FIELDS)
+    _register_late_fee_naming_series()

ns_app/templates/statements/customer_statement.html

@@ -0,0 +1,95 @@
+{# One customer account statement = one printed page.
+   Envelope geometry (window positions in _wrap_document) is field-tuned to the
+   #9 (9x4) double-window envelope. Rendered via frappe.render_template with
+   context key `s` (see ns_app.api.statements.get_statement_data). #}
+{% set fmt = frappe.utils.fmt_money %}
+<div class="statement-page">
+
+  <!-- Return address (top-left envelope window) -->
+  <div class="return-window">
+    <strong>{{ s.company_name }}</strong><br>
+    {{ s.return_address | safe }}
+  </div>
+
+  <!-- Document header (top-right) -->
+  <div class="doc-header">
+    <div class="doc-title">STATEMENT</div>
+    <div><strong>Date:</strong> {{ frappe.utils.formatdate(s.statement_date, "MM-dd-yyyy") }}</div>
+    <div><strong>Account:</strong> {{ s.customer }}</div>
+  </div>
+
+  <!-- Recipient address (lower envelope window) -->
+  <div class="recipient-window">
+    {{ s.customer_name }}<br>
+    {{ s.customer_address | safe }}
+  </div>
+
+  <!-- Statement body (starts below the address windows) -->
+  <div class="statement-body">
+
+    <div class="intro">
+      The following is a summary of your account as of
+      {{ frappe.utils.formatdate(s.statement_date, "MM-dd-yyyy") }}.
+      Please remit payment for any past-due balance at your earliest convenience.
+    </div>
+
+    <table class="items">
+      <thead>
+        <tr>
+          <th>Invoice</th>
+          <th class="c">Date</th>
+          <th class="c">Due Date</th>
+          <th class="c">Days Overdue</th>
+          <th class="c">Aging</th>
+          <th class="r">Outstanding</th>
+        </tr>
+      </thead>
+      <tbody>
+        {% for inv in s.invoices %}
+        <tr class="{{ 'overdue' if inv.is_overdue else '' }}">
+          <td>{{ inv.name }}{% if inv.is_late_fee %} <span class="tag">late fee</span>{% endif %}</td>
+          <td class="c">{{ frappe.utils.formatdate(inv.posting_date, "MM-dd-yyyy") }}</td>
+          <td class="c">{{ frappe.utils.formatdate(inv.due_date, "MM-dd-yyyy") }}</td>
+          <td class="c">{{ inv.days_overdue if inv.days_overdue else "—" }}</td>
+          <td class="c">{{ inv.aging_bucket }}</td>
+          <td class="r">{{ fmt(inv.outstanding_amount, currency=s.currency) }}</td>
+        </tr>
+        {% endfor %}
+      </tbody>
+    </table>
+
+    <!-- Totals -->
+    <div class="totals">
+      <p class="grand"><span>Total Due:</span><span>{{ fmt(s.total_due, currency=s.currency) }}</span></p>
+    </div>
+
+    <!-- Aging summary -->
+    <table class="aging">
+      <thead>
+        <tr>
+          <th class="c">Current</th>
+          <th class="c">1–30</th>
+          <th class="c">31–60</th>
+          <th class="c">61–90</th>
+          <th class="c">90+</th>
+        </tr>
+      </thead>
+      <tbody>
+        <tr>
+          <td class="c">{{ fmt(s.aging["Current"], currency=s.currency) }}</td>
+          <td class="c">{{ fmt(s.aging["1-30"], currency=s.currency) }}</td>
+          <td class="c">{{ fmt(s.aging["31-60"], currency=s.currency) }}</td>
+          <td class="c">{{ fmt(s.aging["61-90"], currency=s.currency) }}</td>
+          <td class="c">{{ fmt(s.aging["90+"], currency=s.currency) }}</td>
+        </tr>
+      </tbody>
+    </table>
+
+    <div class="footer">
+      Prompt payment is always appreciated. We accept payments by check or over
+      the phone using a debit or credit card. Automatic payment setup is also
+      available upon request. Please contact us if payment has already been sent.
+    </div>
+
+  </div><!-- /statement-body -->
+</div><!-- /statement-page -->