Northwind Checkout

Problem

21% of baskets that reached the payment step never completed. Session replays showed the same thing over and over: a card declined for a reason the page never explained, and a customer who left rather than guess.

Worse, a retry after a network timeout occasionally charged twice. It had happened 43 times in a year, and each one cost a refund, a support ticket and a chargeback risk.

Approach

Every write in the flow got an idempotency key derived from the basket, so a retry is provably the same request. That closed the double-charge on its own.

Decline codes stopped being swallowed. Each one maps to a sentence a human wrote — “your bank asked for verification; try again and approve it in your banking app” — and the page keeps the entered card, so a retry is one tap.

The whole flow is three server-rendered steps with the summary as the only shared state. No client-side router, no optimistic anything. A checkout is the wrong place to be clever.

Result

Completed checkouts rose 18.4% within six weeks and held through the following quarter.

Zero double charges since launch, across 2.4 million orders. The support queue for payment issues shrank by roughly two thirds.