Who will pay for this mistake?

Today I went shopping, and I wanted to buy a dress. When I went to the till the sales assistant made a mistake and ended up charging me $2 for a $90 dress. I didn't realize until I got home and looked at my reciept. Obviously, I'm glad I got away with paying a lot less for a dress I really wanted, but I'll feel bad if she has to pay for it out of her wages or something. It was a big brand high street store and the dress was on sale. What do you think? Thanks (UK by the way)