When I buy anything, pet or human, I memorize the advertised price on the shelf, then check the receipt, to make sure they charged the correct price, especially, if the item is marked on sale.
Went to the grocery store tonight, checked my receipt, they charged me $21 for something. Took all the groceries to the cust serv lady, and asked, what is this, that they charged me $21. I knew the only meat I bought, was on sale for $8, and another one for $4, seemed my bill was way higher than it should have been, I am usually pretty close.
Turned out, they charged full price for the one meat item, and, they charged me for 2, I only got one. I seem to come across this pretty frequently, in the grocery store, not being charged the sale price, but first time I got charged for more items, than I actually got.
So, always, check your receipts, before you leave the store. If the person is buying alot of items, they might not notice, being charged wrong, pretty good racket they got, m-o-l.