Undercharged But Over-Appreciated

The national chain store I work in has just closed when we hear someone knocking on the door:

Me: *Through the closed, locked door* “Can I help you?”

Customer: *Frantic and upset* “I need to come in! You made a mistake when you rang me up!”

Me: “I’m so sorry, ma’am, but we’re closed for the night. You’ll have to come back or call in the morning to get it fixed. I’m really sorry we made a mistake.”

Customer: “No! You need to fix it! You undercharged me! I owe you more money!”

Me: “Wait. The mistake was in your favor?”

Customer: “Yes! Please, you need to fix it!”

Me: “Ma’am, I really appreciate your honesty, but if the mistake is in your favor, and it was our fault, you don’t have to do anything else. Don’t worry about it at all. Enjoy your purchase!”

Customer: “Are you sure? I don’t want to get anyone in trouble.”

Me: “Don’t worry about a thing. Our mistake, our problem. Thank you for letting us know and for your honesty, and have a great night!”

Customer: *Much happier* “Okay, good night!”