I don't know what merchant system they are using but I will say it is very common for them to have their own anti fraud detection and rejection on charges. Your specific case could be related to a huge list of possibilities (even many of them a rollup of many other interactions on other sites that happen to use the same merchant or foundational fraud data).

Thinking this is just because of a domain name is silly.

Also Fly.io -- You may want to clearly accept and manage trial issues (or at least a subset) via a support path. It seems silly to effectively bounce users with the impression that you have no support because they are not YET paying customers while in the trial phase.

That would be insane, yes. It isn’t the case, though. I signed up with my own domain just fine.