I am not sure what you are actually asking.
Credit cards have no problem with payments in different currencies. This has nothing to do whatsoever with itch.
If you paid someone with your credit card, it is the terms of your credit card that apply. Look in the terms and conditions of your credit card contract. There often is a fee in % in addition to the currency conversion rate.
Some credit cards can have more than one currency, so if you pay the debt on your credit card, your local bank might have a different (better or worse) fee.