Cash discount is a way to offset all card processing fees. You do this by displaying your card price to the customer. At the checkout if a customer pays with cash they will pay a X% less price than the card cost. If desired, you can do this while not increasing your cash prices. You will need to display your card price in your store. At checkout, input the cash sale price on the terminal/POS. The terminal then automatically increases to the card price by X%. No manual math or input of information is necessary.
*This requires a sign at your store register indicating you are passing the fee or fixed rate to your customer.