diff --git a/backend/__tests__/integration/invoiceDunning.test.js b/backend/__tests__/integration/invoiceDunning.test.js
new file mode 100644
index 00000000..a1d54301
--- /dev/null
+++ b/backend/__tests__/integration/invoiceDunning.test.js
@@ -0,0 +1,110 @@
+/**
+ * Dunning / Mahngebühr logic — the tax-sensitive bits added in the dunning
+ * rework. Covers the fee math (flat / percent), the VAT toggle gating
+ * (incl. the "no-op when the org has no VAT rate" requirement), per-reminder
+ * accumulation (2nd = 1×, 3rd = 2×), invoice immutability (the fee never
+ * changes the issued invoice total), and the 3-reminder cap.
+ *
+ * The Mahnung PDF render is stubbed — PDF rendering (fonts) is flaky in CI and
+ * is verified manually; here we assert the data/immutability behaviour.
+ */
+const { bootCrmDb, seedMinimal } = require('./helpers/crmDb');
+
+let db;
+let cleanup;
+let invoiceService;
+let ids;
+
+async function setSetting(key, value) {
+  const { upsertAppSetting } = require('../../src/utils/appSettings');
+  await upsertAppSetting(key, JSON.stringify(value), 'crm');
+}
+
+beforeAll(async () => {
+  ({ db, cleanup } = await bootCrmDb());
+  ids = await seedMinimal(db);
+  try { await db('customer_accounts').where({ id: ids.customerId }).update({ feature_bills: true }); } catch (_) {}
+  invoiceService = require('../../src/services/invoiceService');
+  // Stub the (flaky) PDF render so applyReminder exercises its data path.
+  // eslint-disable-next-line global-require
+  const pdfService = require('../../src/services/pdfService');
+  pdfService.renderInvoiceToBuffer = async () => Buffer.from('%PDF-stub');
+});
+
+afterAll(async () => { await cleanup(); });
+
+describe('dunning fee resolvers', () => {
+  test('flat fee, no VAT', async () => {
+    await setSetting('crm_invoices_late_fee_enabled', true);
+    await setSetting('crm_invoices_late_fee_type', 'flat');
+    await setSetting('crm_invoices_late_fee_minor', 2000);
+    await setSetting('crm_invoices_late_fee_vat_enabled', false);
+    const inv = { total_amount_minor: 100000 };
+    expect(await invoiceService.resolveLateFeeNetMinor(inv)).toBe(2000);
+    expect(await invoiceService.resolveLateFeeVatRate()).toBe(0);
+    expect(await invoiceService.resolvePerReminderFeeMinor(inv)).toBe(2000);
+  });
+
+  test('percent fee = % of the invoice gross', async () => {
+    await setSetting('crm_invoices_late_fee_type', 'percent');
+    await setSetting('crm_invoices_late_fee_percent', 5);
+    expect(await invoiceService.resolveLateFeeNetMinor({ total_amount_minor: 100000 })).toBe(5000);
+  });
+
+  test('VAT toggle applies the org rate, but is a NO-OP when the org has no VAT rate', async () => {
+    await setSetting('crm_invoices_late_fee_type', 'flat');
+    await setSetting('crm_invoices_late_fee_minor', 2000);
+    await setSetting('crm_invoices_late_fee_vat_enabled', true);
+
+    await db('business_profile').where({ id: 1 }).update({ vat_rate_default: 8.1 });
+    expect(await invoiceService.resolveLateFeeVatRate()).toBeCloseTo(8.1);
+    expect(await invoiceService.resolvePerReminderFeeMinor({ total_amount_minor: 0 }))
+      .toBe(2000 + Math.round(2000 * 8.1 / 100)); // net + VAT
+
+    // Org doesn't charge VAT → toggle adds nothing (Mara's requirement).
+    await db('business_profile').where({ id: 1 }).update({ vat_rate_default: 0 });
+    expect(await invoiceService.resolveLateFeeVatRate()).toBe(0);
+    expect(await invoiceService.resolvePerReminderFeeMinor({ total_amount_minor: 0 })).toBe(2000);
+  });
+});
+
+describe('applyReminder — dunning-document model', () => {
+  let invoiceId;
+  let originalTotal;
+
+  beforeAll(async () => {
+    await setSetting('crm_invoices_late_fee_enabled', true);
+    await setSetting('crm_invoices_late_fee_type', 'flat');
+    await setSetting('crm_invoices_late_fee_minor', 2000);
+    await setSetting('crm_invoices_late_fee_vat_enabled', false);
+    const res = await invoiceService.createInvoice({
+      customerAccountId: ids.customerId,
+      currency: 'CHF',
+      vatRate: 0,
+      lineItems: [{ description: 'Service', quantity: 1, unit_price_minor: 100000 }],
+    }, ids.adminId);
+    invoiceId = res.invoiceIds[0];
+    originalTotal = Number((await db('invoices').where({ id: invoiceId }).first()).total_amount_minor);
+  });
+
+  test('level 2 tracks one fee and leaves the invoice total immutable', async () => {
+    const data = await invoiceService.getInvoiceById(invoiceId);
+    await invoiceService.applyReminder(data.invoice, data.lineItems, 2, ids.adminId);
+    const inv = await db('invoices').where({ id: invoiceId }).first();
+    expect(inv.reminder_level).toBe(2);
+    expect(Number(inv.late_fee_amount_minor)).toBe(2000);
+    expect(Number(inv.total_amount_minor)).toBe(originalTotal); // never mutated
+  });
+
+  test('level 3 accumulates the fee to 2×, total still immutable', async () => {
+    const data = await invoiceService.getInvoiceById(invoiceId);
+    await invoiceService.applyReminder(data.invoice, data.lineItems, 3, ids.adminId);
+    const inv = await db('invoices').where({ id: invoiceId }).first();
+    expect(Number(inv.late_fee_amount_minor)).toBe(4000);
+    expect(Number(inv.total_amount_minor)).toBe(originalTotal);
+  });
+
+  test('sendReminder refuses to exceed level 3', async () => {
+    await expect(invoiceService.sendReminder(invoiceId, 4, ids.adminId)).rejects.toThrow();
+  });
+});
diff --git a/backend/src/services/invoiceService.js b/backend/src/services/invoiceService.js
index 775a5eb1..72f5beb4 100644
--- a/backend/src/services/invoiceService.js
+++ b/backend/src/services/invoiceService.js
@@ -3528,6 +3528,10 @@ module.exports = {
   validateInstallmentPlanInput,
   sendInvoice,
   sendReminder,
+  applyReminder,
+  resolveLateFeeNetMinor,
+  resolveLateFeeVatRate,
+  resolvePerReminderFeeMinor,
   markPaid,
   cancelInvoice,
   releaseForDelivery,