Promocode not working on last page of the shopping cart-how to code?

The promocode is working on all pages of the shopping cart except for the last page before you place the order. Keap rep told me:
I have done extensive testing and the last page that is having the issue w/ the promo code is not originally coded by Keap. The code section that is going to an unknown page needs to be coded to go back to the promo code area.
How can I find instructions on how to do this?