Hacker News, where hackers beg a trillion dollar company to keep not allowing them to create apps on their own hardware without having to go through a bureaucratic process of uploading it to a store. You know, as hackers do.
I wish more people knew about fraud protection and how not to trigger it. If you want to pay in an unusual way, ask support if it's allowed or causes issues.
If you just try and imitate the average customer you will almost always be on the happy path, but ordering multiple books with multiple cards in a short period... I think that would trigger any decent fraud protection and not just at Amazon scale.