It is definitely ethically wrong. I figure that if I speak up when they undercharge me that will stand me in good stead when they overcharge me and I want to speak up about that.
There have been times when I have not. Once I was buying two robes for my twin nephews. They were reduced 75% FROM THE ORIGINAL PRICE which was $49.95 each. This was clearly stated on the sign. Instead, the cashier charged me 75% off the marked down price of $19.96 or $4.99 each, which was a ridiculous price. When I attempted to explain it to her, she pooh-poohed me. I tried again to point to the sign and explain. She then said, “Look, don’t worry about not understanding, not everyone is good at math.” My sister Sue, who had been avoiding catching my eye so as not to burst out laughing elbowed me and said, “Yeah, Marina, not everyone is good at math. Just pay the lady.” If the cashier hadn’t been so obnoxious about my math skills, I might have persevered, but at that point, I took the great bargain.
This story is very much in the spirit of this question and is making big news in Central Florida. I was shocked that most of the people commenting to the newspaper thought the woman’s actions were not wrong!