diff --git a/CHANGES.md b/CHANGES.md
index 9c38282..cfc676b 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,6 +1,7 @@
 # Changes
 
 ## Unreleased
+- Added `checkout(items, { coupon })` returning `{ subtotal, discount, tax, total }`, with SAVE10 and FLAT5 coupons.
 
 ## 1.2.0
 - total() rounds through roundCents().
diff --git a/src/lib/cart.js b/src/lib/cart.js
index 385fd69..d5f21ef 100644
--- a/src/lib/cart.js
+++ b/src/lib/cart.js
@@ -8,3 +8,29 @@ export function subtotal(items) {
 export function total(items) {
   return roundCents(subtotal(items) * (1 + TAX_RATE));
 }
+
+const COUPONS = new Map([
+  ['SAVE10', (sub) => roundCents(sub * 0.1)],
+  ['FLAT5', (sub) => Math.min(5, sub)],
+]);
+
+export function checkout(items, { coupon } = {}) {
+  const active = items.filter((item) => item.qty > 0);
+  const sub = roundCents(subtotal(active));
+
+  const code = (coupon ?? '').trim();
+  let discount = 0;
+  if (code !== '') {
+    const apply = COUPONS.get(code.toUpperCase());
+    if (!apply) throw new Error(`Unknown coupon: ${code}`);
+    discount = roundCents(apply(sub));
+  }
+
+  const tax = roundCents((sub - discount) * TAX_RATE);
+  return {
+    subtotal: sub,
+    discount,
+    tax,
+    total: roundCents(sub - discount + tax),
+  };
+}
diff --git a/tests/lib/cart.test.js b/tests/lib/cart.test.js
index 7a9eab3..3a18f3a 100644
--- a/tests/lib/cart.test.js
+++ b/tests/lib/cart.test.js
@@ -1,6 +1,6 @@
 import { test } from 'node:test';
 import assert from 'node:assert/strict';
-import { subtotal, total } from '../../src/lib/cart.js';
+import { subtotal, total, checkout } from '../../src/lib/cart.js';
 
 const items = [{ price: 10, qty: 2 }, { price: 5, qty: 1 }];
 
@@ -11,3 +11,51 @@ test('subtotal adds price times quantity', () => {
 test('total applies tax and rounds to cents', () => {
   assert.equal(total(items), 27);
 });
+
+test('checkout without coupon applies tax only', () => {
+  assert.deepEqual(checkout(items), { subtotal: 25, discount: 0, tax: 2, total: 27 });
+});
+
+test('checkout treats empty or missing coupon as no discount', () => {
+  const expected = { subtotal: 25, discount: 0, tax: 2, total: 27 };
+  assert.deepEqual(checkout(items, { coupon: '' }), expected);
+  assert.deepEqual(checkout(items, { coupon: '   ' }), expected);
+  assert.deepEqual(checkout(items, { coupon: null }), expected);
+});
+
+test('checkout SAVE10 takes 10% off before tax', () => {
+  // 25 - 2.5 = 22.5; tax 1.8; total 24.3
+  assert.deepEqual(checkout(items, { coupon: 'SAVE10' }), { subtotal: 25, discount: 2.5, tax: 1.8, total: 24.3 });
+});
+
+test('checkout FLAT5 takes $5 off before tax', () => {
+  // 25 - 5 = 20; tax 1.6; total 21.6
+  assert.deepEqual(checkout(items, { coupon: 'FLAT5' }), { subtotal: 25, discount: 5, tax: 1.6, total: 21.6 });
+});
+
+test('checkout FLAT5 never discounts more than the subtotal', () => {
+  const small = [{ price: 3, qty: 1 }];
+  assert.deepEqual(checkout(small, { coupon: 'FLAT5' }), { subtotal: 3, discount: 3, tax: 0, total: 0 });
+});
+
+test('checkout coupon codes are case-insensitive and trimmed', () => {
+  assert.deepEqual(checkout(items, { coupon: '  save10 ' }), checkout(items, { coupon: 'SAVE10' }));
+  assert.deepEqual(checkout(items, { coupon: 'flat5' }), checkout(items, { coupon: 'FLAT5' }));
+});
+
+test('checkout ignores items with qty 0 or less', () => {
+  const withJunk = [...items, { price: 100, qty: 0 }, { price: 100, qty: -2 }];
+  assert.deepEqual(checkout(withJunk), checkout(items));
+});
+
+test('checkout throws on unknown coupon with trimmed code in message', () => {
+  assert.throws(() => checkout(items, { coupon: '  BOGUS ' }), { message: 'Unknown coupon: BOGUS' });
+  assert.throws(() => checkout(items, { coupon: 'constructor' }), { message: 'Unknown coupon: constructor' });
+});
+
+test('checkout rounds each money field to cents', () => {
+  const odd = [{ price: 0.99, qty: 3 }];
+  const result = checkout(odd, { coupon: 'SAVE10' });
+  // subtotal 2.97; discount 0.297 -> 0.3; taxable 2.67; tax 0.2136 -> 0.21; total 2.67 + 0.21 = 2.88
+  assert.deepEqual(result, { subtotal: 2.97, discount: 0.3, tax: 0.21, total: 2.88 });
+});