You will have to cancel/refund the transaction with the buyer's agreement (via the dropdown menu arrow for the item on your orders list). Then relist for them to buy again, in a category that allows you to use the Oversize option so you don't have to use Simple Deiivery. Unfortunately we don't have a list of categories that allow that so it's trial and error.
So relist and change category or sub-category until you get "Oversized" at the bottom of the weight pop-up under the Package size heading, during the listing process. Then click on Add Primary Service to buy your own postage.
