Compare that to the ~30% I've seen, that's sadly an amazing shift (lol, which won't ever happen with the fascist caucus), but I commend Bernie for trying.
Iirc, surcharging is only illegal in 4 states. However, theres a loop hole in thise states, giving a discount on cash purchases arent considered a surcharge...
Processing or transaction fees. Anytime you use your card for a purchase the bank gets a cut of that. This fee can range from .1% to 4%, depending on the credit card processor.