Parking cost hidden fees?

I went to the game yesterday and parked in Lot C like I always do. Parking is $30 now (damn inflation). They only take credit cards now, no cash – ok fine. But I noticed something odd when I checked my balance this morning. They didn't charge $30, no, they charged $33.98. I know it's only $3.98 difference, but there was no warning. There's nothing stating that you have to pay a processing fee or merchant fee or anything else above the $30. This seems shady, maybe even illegal. Does anyone have any insight to this?