You don't, actually. It's illegal to charge an additional fee for using a card. It's legal to offer a discount for cash, however. Not only that but the fees charged per card are different depending on the card and the banking network it's processed through. The debate about coffee shops taking bitcoin is all about the access, as per usual. If Square ran on BTC your local coffee shop would run on BTC. The problem is that BTC is becoming cumbersome for anything but big transfers; it's starting to look more like a bad implementation of ACH and less like a peer-to-peer network (more Spotify, less Pirate Bay). If you're going to put up with something cumbersome, why not stick with the FDIC-insured, not-taxed-as-property alternative known as "money?"