If a shopkeeper overcharges you, they ask you to pay too much for something. To overcharge isn't very honest, but it's not usually illegal either.
Sometimes it's obvious that someone is trying to overcharge you: if the taxi driver announces that you owe her a hundred dollars after driving a route that usually costs about twenty, you can call that a blatant overcharge. In other cases, you may not realize until much later. It's not uncommon for salespeople to overcharge tourists, for example, making them pay too much money because they don't know any better.