American Express costs more because they charge a flat rate without downgrading. Visa, MasterCard and Discover use interchange, so they begin with the lowest rate possible and then charge penalty rates depending on the transaction. Let’s say one of your customers wants to buy a shirt and they want to pay ahead of time, they call you they give you their credit card over the phone. If you accept that transaction, you will get a penalty, a downgrade and thus charged more because it was not done face-to-face. American Express charges one rate regardless of how you process the transaction.