I don't think you were out of line. When I worked retail, if I someone made a mistake in the company, ESPECIALLY if someone ELSE made a mistake and I had to set it right, I was extrememly polite and helpful when the person came back to have their problem set straight. And if there was even a $1 difference, I'd have not said anything, speicifically because the poor person had ot schlep their problem back and forth.

But, all that said, my daughters would have said the same thing. I would have reacted the same way andI'd have been told I was rude. I don't think its rude when you are merely pointing out their lack in customer care.