Merchants pay transaction fees like 2-3% on the transaction.
But credit companies make miney from issuing credit. As high as 20-25% APR.
What nobody here ever have a credit card? Lol
The APR/interest that is charged by banks is to account for the credit risk. The actual returns is not as high because banks generally suffer heavy losses when lending via credit cards.
The actual issue is the price that merchants pay in exchange for having credit card payments processed. You are generally correct as to the percentage, however many merchants will also pay some fixed amount above this percentage.