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@@ -42,7 +42,12 @@ services/checkout.ts
  async function checkout(order) {
-   const total = order.items.reduce((s, i) => s + i.price * i.qty, 0)
+   const total = computeTotal(order.items)
+   if (!Number.isFinite(total)) {
+     throw new Error('invalid order total')
+   }
    return paymentService.charge(order.customerId, total)
  }