Suppose the sign is wrong, because it says 5 guaranteed epics, when it actually gives 1. ok
Now, when you buy something, the merchant, the seller has the obligation to deliver to you what it says on the sign and at the price it says there! If the seller was wrong, bad luck for him, but he has the obligation to give you that he says!
And this is not different. When you buy something on the internet, you have the right to claim what has been offered.