[wstuartcl]

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.