Had this issue. The error messages varied, from failed credit check due to fraud to bad card information. Retried many times, finally just used another card.
It is surprising that whatever load testing this system underwent this still happened.
I don't give much leeway to Apple on this. No, "well, goldman sachs..." or "citizens wouldn't do..." whatever the issue was, Apple should have handled it by doing their own mitigation.
This is possibly the world's most powerful company, handling orders for its most valuable product using its own financial product. Get it together.