Answer:
First, I would check and see if she was correct. If she wasnt, I would simply show her why she was incorrect. If I was to accidently short her, I would give her the money and move on with my day. if she refused to leave after showing her that I gave her the correct amount of money, she would probably call for the manager. I'd get the manager, explain what happened, and the annoying customer would most likely get kicked out of the store.
(P.S: sorry if it was a little confusing lol)