Durbin Amendment
The Law states, in the Durbin Amendment (part of the 2010 Dodd-Frank law), businesses are permitted to offer a discount to customers as an incentive and to encourage customers to pay by alternative methods other than a credit/debit cards including either checks or cash in order to automatically receive a discount which is applied at the time of sale.
However, it is illegal to make money on anyones payment method, therefore you cannot charge more than you are being charged by your provider.