I have been overcharged in my LQS and i realized it when I got to the car. I took the receipt back in with the fabric and we went through it and she fixed the problem. I have also been undercharged and I did the same thing. It was a $20 mistake in my favor. The owner was so glad I was honest. It is bad Karma not to tell people when they have made a mistake especially when you are cheating someone.
Since has happened a couple of times, I try to add in my head approximately what the bill is going to be. If it is not close I just say "that does not sound right (either too much or not enough)". They make mistakes but most of them are very gracious about it.
If the gentleman does not own the store, I would let the owner know what happened because they could be losing business and that is tough in this economy. If he owns the store, I probably would shop else where if I had other options.