This happened to me a few times at 7-11's frequented by tourists in Pattaya. When they see that you don't pay attention to how they scan the items or are talking to someone or in a rush, they may inflate the bill. And of course they won't give you a receipt. Even when I asked for it, she lied some BS. Has this happened to you?
Now I want to catch them red handed. I will pick lots of stuff and check the prices and then pretend that I am in a rush or talking on the phone and don't care about what they are doing. And then confront them with clear evidence.