The Cash Discount program may be confusing to some customers. Here is a simple way that you could explain it.
"Prices on our menu reflects a Cash Discount of 3.99%, so you are saving instantly if you pay with cash. If you pay by credit or debit card, you will not be taking advantage of the 3.99% cash discount and that difference will be added to your bill. This 3.99% covers the card processing fee."