Uneven Understanding Of Even Exchange

(I explain to a customer I must return her damaged online item in a separate transaction from her buying the new one. It must always must be done separately.)

Customer: “OH, YEAH, THAT’S FINE!” *almost jubilant*

Me: “Okay.” *somehow knowing it won’t be*

(I return the item to her card, explaining the process as I am going. I ring up the replacement item, which is now even cheaper than she originally bought it.)

Customer: “WAIT. I DON’T GET IT. IT’S AN EVEN EXCHANGE. I SHOULD HAVE TO PAY NOTHING!”

Me: *explains it several times until she gives up and pays and goes away*