How to use a radio button to add a discount to a pdf form calculation.

I am building a form for my company's sales team. I need to be able to add a cash discount should the customer choose this payment option. I was thinking of using a radio button to simply apply the discount for the sales person. However, I don't know how to write the code for this. How would I make this work without making the form overly complicated for my sales team? Or is there a simpler option that I could use?