They need a “no problems line”. It would work like this: You have a problem with your purchase, you would have a choice, Pay full price and go to customer service to fix it or not get the product. Coupon doesn’t work, that’s a problem. Ring up price doesn’t match the shelf price, that’s a problem.
Also if you are over the limit for the express line, you would forfeit the balance of your purchases, no argument! 21 items in the 20 or less line, you are not getting the 21st item, Period!