Be Polite and don’t come back. That’s my rule of thumb.

Meanwhile on the other side of the coin, people have literally been shot and killed for having an extra item in their bag that they didn’t pay for.

… but they WEREN’T doing their job. I’ve been a counter cashier at a burger joint. Most of the job was getting the order correct and taking in money properly, but I also has to to things like add extra relish packets and see that I was giving them the correct food. That’s the job. You give the customer what they ordered. That is the EASY part. The hard part is dealing with the people trying to scam you with bill-switching/wrong-change schemes (though I suspect those are less common as fewer people use cash now).