I started a reply here simply to comment that Stripe doesn't do currency conversion, they just charge in USD.

But I double-checked their FAQ and couldn't verify that, actually... I can't find details on how they manage the actual charge to foreign cards.

It's an important issue, because if they just charge in USD, holders of foreign cards will presumably see a "foreign transaction fee" in their statement, which discourages future purchases.

If they are able to charge the card in its own currency and avoid the fees, that means they're doing the conversion themselves, and setting the rate.

I still suspect it's the former solution, but I'll have to ask about that....

Shouldn't be too hard to create a script that sends a Stripe form to US customers (either based on country selection or just IP) and a Paymill form to EU customers.