Each one is a service that passes its tests and still fails in production. Pick one.
Some customers are billed twice for one order. The visible tests pass. Work out what production does that the tests do not.
One checkout is charged once and creates one order, however many times it arrives, even while the first attempt is still being paid.
Every repeat of a checkout gets the same answer and the same order as the first attempt.
A customer may buy the same item twice on purpose, with the same card.
After a declined card, the customer can pay for the same checkout with another card.
Different checkouts must not wait for each other.