It never fails when you're leaving Costco. You get the receipt from the checkout, shove it into your pocket, and then go through the dance of trying to fish it out so the person standing at the exit can draw a line on it. But why is this so important?
You probably expect it's done as a loss prevention measure, but according to Costco's own "standard practice" policy, the warehouse store does it for a totally different reason.
The employees are actually checking to make sure that customers were charged the correct prices for the items and that they were properly scanned. Some former employees admit it happens quite a bit.
But don't try leaving the store without that mark. In the Costco membership application terms, members agreed to abide by the rule.