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.
If you have problems with that the alternative is to list with free postage, factor that cost into your item price and buy your own postage. The payment may be held for 14 days.
Evri takes parcels that long. Google for more info.
