The saga of the forgotten coupon

Mobile poster, so apologies for formatting. I work at a charity thrift shop. Yesterday was a very busy sale day, as all of our Mondays are. Every Monday, the color tag that was half off the previous week, and incidentally also the oldest color tag, becomes one dollar. This goes for everything in the store. Clothes, books, furniture. Everything. People like to take advantage of it, and get tons of great stuff for cheap. Mostly clothing though, tbh.

Anyways, this one lady comes in my lengthy line with a cartload of clothing. I get through the transaction very slowly, since she insists on putting one item up at a time, and I have to scan it and remove the hanger. She also decided this is the best time to sort through her selection and decide if she actually wants any of them, rather than do it while she was shopping or on line. She gets done with the transaction, I ask if she wants to donate or has any coupons or discounts, she says no. Total was like $103 USD. Not the biggest transaction of the day, but big enough to be notable. She leaves, and comes back later with a question. "I forgot I had a 20% off coupon to apply to my transaction, is there any way you could put that on?" "I'm sorry ma'am, I can't apply discounts retroactively."
Technically, I can, I just have to refund everything, apply the discount, and apply the remaining balance to a gift card. She leaves, and comes back again asking for a manager, who I call up.
After talking to the manager, who informs me that I should refund her, return everything the discount can apply to, apply the discount, re-ring everything up, and apply the balance to a gift card, as we don't offer cash returns. I do all of this, all the while as a line builds up. She then thanks me, and gives me a dollar for the trouble, which isn't all too bad.