Honestly, and maybe it's because I work retail, but it drives me crazy to hear people say things like that.
"You can pay for mine too- ha ha!"
"You have enough money; you buy this time!" "If it doesn't scan, it's free!" And so on ad nauseam.
Any jokes about money like that annoy me, but I just play along. But it's just kidding around and the person certainly meant no harm. Why get so worked up over it?
I've started catching myself before I say that, I've heard that all cashiers just get so annoyed hearing it all day. I try to cut back, some times it works. Not always, though...sorry cashiers out there!
Don't beat yourself up over it
When I was a cashier, I would have much rather had a friendly customer using a joke I'd already heard 100 times that day, than a rude, SS customer. Yes, it gets old (quickly), but there's far worse things.
For example, don't pay entirely in pennies. And especially, don't send your young child through the line to buy a $25 item in small change (dimes and lower) to teach them how to save/pay for themselves. Especially if you're not going to roll the money at home OR help them count it at the register.