My brother's covered contract law for his degree. He reckons that when you bought the books, it was you offering a price. As they accepted the money, crucially, when the money was exchanged, the contract was sealed - in this case, they would be bound now to supply you with the items for the agreed price. That is indeed contract law! So basically, if you could find a book that said that (take one off the shelf in the shop for added insult) and showed them, you can say that 'it says here, 'tough!''