In most cases, it does not cost you anything to return mail to the sender. You can return it to your mailbox or place it in a collection box without any new postage. If you have already opened the package, then you will probably have to pay to make the return. The amount of postage required will depend on the size and weight of the package you are returning. A standard first class envelope might only require a single stamp while a larger package might require $10 or more of postage.