If you rely on http to ensure the one-and-only-one type of paradigm, I have bad news for you (because you won't be able to distinguish failure vs success case when you hit a timeout: did it not receive the request or did I not receive the ack?).

You should embed a transaction ID to ensure idempotence of your API call. Then you can make the request 1, 2 or 3 times without any risk. The receiving end will realize the transaction ID already transitioned state and won't cause multiple payments to occur.