Originally Posted by
Chihaya
Here, the processing fee is paid by the business sellers, in this case - SE, not you.
As a consumer, unless you get a card with annual fee or minimal usage fee, you don't have to pay anything unless you keep an unpaid balance.
I pay my cards off at the end of the month, have yet paid AMEX/VISA/Discover anything extra other than foreign transaction fees, which is about 2.5~3%.
They stay in business by those processing fees from the merchants and collecting interest of unpaid balances.