usually a percentage of the amount this of the sale.
It's usually a percentage plus a fixed amount. Rewards cards and AMEX and Discover can be significantly higher than your bog standard bank account card.
Here's Squares rates, which flatten out all the variables to these standard fees. Traditional merchant credit card processing services pass on the exact charges based on the type of card, issuer, and entry method. I think AMEX was 3.5%+.
The Square standard processing fee is 2.6% + 10¢ for contactless payments, swiped or inserted chip cards, and swiped magstripe cards. Payments that are manually keyed-in, processed using Card on File, or manually entered using Virtual Terminal have a 3.5% + 15¢ fee. Invoices cost from 2.9% + 30¢.