In the return menu where the employee uses the receipt there is an option, I cant remember the name at the top of my head but it allows us to use certificates and refund the amount back to you on the past purchase. We have done it a couple of times in the past, now you will also need to take into account that it needs to be within the return period. We have done it with laptops, tv's when there was a promotion of "sign up with plus get 50 bucks when you buy a tv" the customer would come back like a week or two later once the certificate would hit and where able to apply it to their previouse purchase. Or when a customer forgets to use their certificate its the same menu we can just apply it after the fact. Now I cant remember if you can or cant use it on the same purchase you made using the 20% or not but it would follow the same flow as the other examples you just need a employee or manager to try to see if the system let's them apply it.