I had a similar situation recently and I handled it similarly to the way asyyeger said. I had accidentally taken a cottage cheese with olives to the checkout, and as she was ringing it up, I noticed that it was the wrong one. (I had meant to buy one without olives.) I told the cashier and she said that I should just go back and take the one I wanted and she would meanwhile ring it up so I didn’t have to go through the line again. I went back, took the cheese, and on my way out, held it up to the cashier and said, “Thanks, I got it.” Better not to take chances of chillul Hashem.
(That said, had I already left the store, I wouldn’t have gone back to explain. I think that would have made you look really strange.)