[idupree]

I prepay for many services that are still able to list prices in real currency:

https://www.fastmail.fm/

https://www.tarsnap.com/

https://www.nearlyfreespeech.net/

The latter two do keep track of fractional charges. They deduct $0.01 from prepaid credits once that much is owed. Since you accept multiple currencies, this becomes a bit more complex but still doable. For example, let the user choose the currency to display, or pick a single standard currency (euros?) and use it as your units.

Tarsnap and NFSn choose to refund your prepaid credits if and only if you close your account with them. Not every prepaid system lets you get your prepayments back at all. I am fine with this: I am willing to pay a bit more (prepayment without refund) in order to limit my liability to the amount I intend to pay.