This is really the best answer, I think. Except the agreement you used to have to sign to take Visa cards used to stipulate that you could not offer a discount for cash.

Visa’s rules have changed in favor of the merchants at last….. Visa has updated their V isa Core Rules and Visa Product and Service Rules, and the rules changed benefit all business owners in all 50 states ! …Now, you can offer a discount when your customer pays with cash in efforts to incentivize your customers and avoid having to pay credit card processing fees. (Therefore if you sell a product or service for $100, you get to finally keep all $100 of the sale….Not $96.00 )