Cancelled Product I paid for

I went onto [URL Removed] and bought a screen for price x. I paid for the item and was waiting for delivery. I saw that the screen i paid for was cancelled?. Their email: o we so sorry but that was the incorrect price? So what does that have to do with anything. You advertised it and then I paid for it and now you like oops no screen for you. If i was in a shop I would have walked out with a screen. It is because they still have to deliver that they decide if they want to give you the product? So if an item is on sale and you pay for it they can decide if they want to sell it at that price or not? Me paying for an item does not mean that it is the final price of the item? I am so disappointed in them and i will never buy anything from them again.