You can just cancel the subscription at any moment. I had American friends doing that.
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.