When I lived in NE Portland a very small market, a block away, would advertise great buys on a large sign on their street side window. Almost every time I purchased items the price I was charged was incorrect. At first I wondered if I’d remembered seeing the price on the shelf wrong but when I was charged more than any other store in town for apples, and they were the special of the week and advertised on the window sign I knew I’d remember the price correctly. I went back in.
The clerk looked surprised EVERY TIME I returned with this information. He’d insist the price on the receipt was right, I’d point to the sign, he’d be surprised it was there, have to decide whether he should honor it or not, be flustered, I was causing problems but he was stoic, he took a long time with the math, and do things on the cash register, redo them, and recount the money. He’d get angrier by the minute because I was holding up the customecurs in line. I understand he was worse to any uppity kid who could do math.
In my book Gracie’s Grandfather Makes Trouble, Grace and her Grandad confronted this clerk but had an interesting solution to the problem.  nancymauerman.com