Managing returns can be a real hassle for the seller, especially in cases of buyer remorse or when a returned item has been damaged by the buyer. In situations such as these, you may charge a reasonable restocking fee. But how much is "reasonable"?
Typically, a restocking fee is a percentage of the item's price. The percentage depends on the type of item and the condition in which it is returned. Amazon allows sellers to charge restocking fees only in well-defined situations.
For more information on when you can charge a restocking fee and how much you can charge, search on "managing returns" in Seller Central Help. Also refer to the Amazon Product Return Policies.