Can I add in a promo code for my payment options?

I would like to add in a form to allow users to enter in a promotional code for a discount.. is this possible? 

Thank you!

Yes, that is possible and quite simple.

What I would suggest is to check out the following steps: How To Add Coupon Code in the Payment Form

You will be able to see how to set up your coupon codes and the field for accepting the same will be added to your payment processor by itself.

Do of course let us know if you have any questions along the way and we would be happy to assist with the same.