Simple Delivery covers most items for up to £750.    For an extra large item you should be able to opt out of SD if you like and buy your own postage.

The postage options available depend on category.   So 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.   It has to be done on a laptop or desktop rather than the app.