You'd have to cancel the transaction with buyer's agreement (via the dropdown menu for the item on your orders list)  and relist.      Then to opt out of Simple Delivery and buy your own postage you have to change category or sub-category until you get "Oversized" at the bottom of the weight pop-up under the Package size heading.   Click on Add Primary Service.    It has to be done on a laptop or desktop rather than the app.