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. To mark dispatched and get buyer's address you still need to open the original prepaid label (Get prepaid label > print label). It will remain unused.
The option to change the tracking from Simple Delivery has been removed but you would be able to add the new number to a "not received" case if that were necessary.
You can claim a refund for an unused label here:
https://www.ebay.co.uk/claims/simple-delivery/refund/seller
Without tracking on ebay the payment may be held for 14 days.
@igr1648
