I don’t think 32 vs 64 bits is correct. I think it is F-Droid signed vs developer signed (reproducible build).
Every month I am answering several thousands of questions, and I guess it would surprise you what types of questions I get. Saying that it is ‘user error’ is too short-sighted, also because the developer has a responsibility too. Anyway, I am rather careful to not get more questions, and this might be a reason to not go for a reproducible build.
The Play Billing API is proprietary, but it won’t and can’t do anything if there are no Play Services on the device because there is no service to interface with. So, the benefit of having a reproducible build is limited anyway.