Huh?

A girl approaches the register and hands me a book.
I ring it up, say "$15.75" and she hands me a gift card for $15. "Okay, 75 cents," I say.
"I don't got 75 cents."
"Well, you can use a credit card!"
"Don't got one."
Well.
"I'm not...uh, okay...?" I void the transaction and hand her back the card. Cause I'm really confused.
She looks at the card and at the book that I've set on the re-sort cart.
"So I don't get the book?"
Um? I'm not understanding what she's not understanding about how stores work. You can't bargain with me. Does she think I'm going to give her 75 cents to buy what I'm SURE is an excellent book. Entitled "A Hustler's Wife," it is the story of how young Yarni leaves home and gets involved with--wait for it--a hustler.
"No...I can't complete the transaction without the full tender."
"Can I get a different one?"
Duh.
"You can buy anything you want under $15."
She rolls her eyes at me and walks away.
Don't roll your eyes at me! You're the one who doesn't understand how a basic transaction is performed.
She owes ME 75 cents in stupidity points.