diff --git a/CHANGES.md b/CHANGES.md
index 9c38282..7859cf4 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,6 +1,7 @@
 # Changes
 
 ## Unreleased
+- Added checkout(items, { coupon }) returning subtotal, discount, tax and 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..aee7e1d 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 live = items.filter((item) => item.qty > 0);
+  const sub = roundCents(subtotal(live));
+
+  const code = (coupon ?? '').trim();
+  let discount = 0;
+  if (code !== '') {
+    const rule = COUPONS.get(code.toUpperCase());
+    if (!rule) throw new Error(`Unknown coupon: ${code}`);
+    discount = roundCents(rule(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..76adb2f 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 +11,72 @@ test('subtotal adds price times quantity', () => {
 test('total applies tax and rounds to cents', () => {
   assert.equal(total(items), 27);
 });
+
+test('checkout without a coupon charges tax on the full subtotal', () => {
+  assert.deepEqual(checkout(items), { subtotal: 25, discount: 0, tax: 2, total: 27 });
+});
+
+test('checkout ignores items with qty 0 or less', () => {
+  const cart = [{ price: 10, qty: 2 }, { price: 99, qty: 0 }, { price: 99, qty: -1 }];
+  assert.deepEqual(checkout(cart), { subtotal: 20, discount: 0, tax: 1.6, total: 21.6 });
+});
+
+test('checkout of an empty cart is all zeros', () => {
+  assert.deepEqual(checkout([]), { subtotal: 0, discount: 0, tax: 0, total: 0 });
+});
+
+test('SAVE10 takes 10% off the subtotal before tax', () => {
+  assert.deepEqual(checkout(items, { coupon: 'SAVE10' }), {
+    subtotal: 25,
+    discount: 2.5,
+    tax: 1.8,
+    total: 24.3,
+  });
+});
+
+test('FLAT5 takes $5 off the subtotal before tax', () => {
+  assert.deepEqual(checkout(items, { coupon: 'FLAT5' }), {
+    subtotal: 25,
+    discount: 5,
+    tax: 1.6,
+    total: 21.6,
+  });
+});
+
+test('FLAT5 never discounts more than the subtotal', () => {
+  const cart = [{ price: 3, qty: 1 }];
+  assert.deepEqual(checkout(cart, { coupon: 'FLAT5' }), { subtotal: 3, discount: 3, tax: 0, total: 0 });
+});
+
+test('coupon codes are case-insensitive and ignore surrounding spaces', () => {
+  assert.deepEqual(checkout(items, { coupon: '  save10 ' }), checkout(items, { coupon: 'SAVE10' }));
+  assert.deepEqual(checkout(items, { coupon: 'flat5' }), checkout(items, { coupon: 'FLAT5' }));
+});
+
+test('empty or missing coupon means no discount', () => {
+  const plain = checkout(items);
+  assert.deepEqual(checkout(items, { coupon: '' }), plain);
+  assert.deepEqual(checkout(items, { coupon: '   ' }), plain);
+  assert.deepEqual(checkout(items, { coupon: undefined }), plain);
+  assert.deepEqual(checkout(items, {}), plain);
+});
+
+test('unknown coupon throws with the trimmed code in the message', () => {
+  assert.throws(() => checkout(items, { coupon: '  nope ' }), {
+    name: 'Error',
+    message: 'Unknown coupon: nope',
+  });
+});
+
+test('coupon names that exist on Object.prototype are unknown coupons', () => {
+  assert.throws(() => checkout(items, { coupon: 'constructor' }), {
+    name: 'Error',
+    message: 'Unknown coupon: constructor',
+  });
+});
+
+test('subtotal, discount and tax are rounded to cents', () => {
+  // 0.1 * 3 is 0.30000000000000004 in floating point; tax on 0.3 is 0.024.
+  const cart = [{ price: 0.1, qty: 3 }];
+  assert.deepEqual(checkout(cart), { subtotal: 0.3, discount: 0, tax: 0.02, total: 0.32 });
+});