However, reportedly a small number of apps allow paying by credit card. And some are more flexible regarding cancelling the subscription than others.
I don't want to become an expert on buying the optimal set of tickets from multiple regions.
Having the ticket would help a lot navigating around without fear of having the wrong ticket or making dealing with delays or cancellation even more frustrating.
For buying it, ID might be required at some sellers, but typically it's not.
It's probably not impossible, but it's also not as simple a transaction as it could be. It is a difficult affair, at least bordering on impossible unless you're a EU citizen with a SEPA bank account.
However, another comment somewhere mentions a seller that accepts Paypal. I'm sure I have heard about others accepting credit card. They are rare, but they exist. It doesn't matter where in Germany you buy the ticket, it's always valid everywhere. Buying the smart card version at some weird place might be practically infeasible and sometimes they are not issued on the spot but after a delivery time. But for buying the app version you "just" have to do your homework. Quick, easy, and convenient, no. Possible, yes.