Retry safely with an Idempotency-Key

If your request times out, you cannot know whether we received it. Retrying can charge twice, unless you send an Idempotency-Key header.

1. Make one random key per action you want done once, such as one per order. A UUID works well.
2. Send it as the Idempotency-Key header every time you try that action.
3. If we already did it, we return the original answer and do not charge again.

Create the key when the customer presses the button, store it with the order, and reuse it for every retry of that order. A new key means a new payment.