If i buy a game that's 5 dollars but I use a different currency from another country, how much am I charged? is the conversion rate for itch.io different?

Itch doesn't allow you to change the currency rate.

If an amount is in dollars, then you must pay the dollar. In these cases, your local payment method performs the conversion and determines the exchange rate. Therefore, you should ask the company that provides your payment method, not the store.