I shrug leave the cart there and start walking out the door.
No, wait: *The cashier goes and opens the closest register to the self checkouts
Honestly this would piss me off more. Oh so you just lied to me and expect me to forget about it that quickly. If that's how they treat customers I wouldn't ever return. There's too many options for me to put up with that bs.
I know not everyone has options, but exercise them if you do.