I paid once and gave the cashier extra ones to get a bigger bill back as change and she was trying to give them back. I said just punch in the numbers and you'll understand.
Tell them it's magic and watch their eyes light up when a few low value coins turn into two quarters or a couple of ones into a five or ten.
"If the register does not add it up, figure the tax, and make the change they can't do it."
Tax charts are a thing of the past, I can only think of one place I've seen one in the last 5 years and that's at the mom and pop hardware store. I handle money and I have to admit it does take me a few seconds to figure things out if I ring in the sale wrong. Computers have made us lazy.
YOU should have known better. :)