Trying To Discount The Idea Of A Discount

(This story was told to me by my coworker. I witness the interaction happening but don’t understand what is going on at the time. A customer purchases two light bulbs at a discount and comes back the next day to return them.)

Coworker: “You’ll have $14.50 credited back to your card.”

Customer: “No, the bulbs cost $10 each; it should be $20.”

Coworker: “Yes, but it shows here on your receipt that you were given a discount.”

Customer: “Yes, but the bulbs cost $10, so I should get $20 back.”

Coworker: “You only paid $14.50 for the bulbs.”

Customer: “Yes, but they cost $10.”

Coworker: “I can’t give you back $20 for something you paid $14.50 for.”

Customer: “Why not?”