We have a written contract with an author to write a book about famous dogs. We agreed to pay the author a $50K advance. The written contract says it is the “full and final expression” of the agreement. Today the author claims we were also supposed to pay her travel expenses to take pictures of the famous dogs, and she sent us some receipts for airfare and hotels. There’s nothing in the written contract about paying these travel expenses. All we have is an email dated before we signed the contract saying we might consider paying them if they are reasonable. Can the author demand payment of the expenses? Is there any legal rule that might protect us from having to pay?