Initial import of zappier-edge from zapier monorepo

tests/credit.test.ts

@@ -0,0 +1,33 @@
+import { applyMonthlyCredit } from '../src/billing/credit';
+import { DEFAULT_TIERS } from '../src/pricing';
+import { UsageSummary } from '../src/usage';
+
+const freeTier = DEFAULT_TIERS.find((t) => t.id === 'free')!; // monthlyCreditCents: 100
+
+const summary = (totalCents: number): UsageSummary => ({
+  customerId: 'cust_1',
+  totalCents,
+  calls: 1,
+  byEndpoint: {},
+});
+
+describe('applyMonthlyCredit', () => {
+  it('covers usage fully when under the monthly credit', () => {
+    const billed = applyMonthlyCredit(summary(60), freeTier);
+    expect(billed.includedCents).toBe(60);
+    expect(billed.billableCents).toBe(0);
+    expect(billed.totalCents).toBe(60);
+  });
+
+  it('bills only the overage when usage exceeds the credit', () => {
+    const billed = applyMonthlyCredit(summary(250), freeTier);
+    expect(billed.includedCents).toBe(100);
+    expect(billed.billableCents).toBe(150);
+  });
+
+  it('bills nothing when there is no usage', () => {
+    const billed = applyMonthlyCredit(summary(0), freeTier);
+    expect(billed.includedCents).toBe(0);
+    expect(billed.billableCents).toBe(0);
+  });
+});