Whether or not the "Oversize" option (which you choose in order to opt out of Simple Delivery and buy your own postage) is available seems to depend on the category you use. So during the listing process select category and enter item title, scroll down and click on the box under the heading Package Size. At the very bottom of the pop-up you should see:
Oversized
Above 20 kg or 61 x 46 x 46 cm
Select that and you should get:
Domestic postage
Simple Delivery is unavailable for this item
If you're not getting that change the category or sub-category until you do. Once you get to Oversized 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 and factor that cost into your item price. To get the buyer's address and mark dispatched you need to open the original prepaid label (Get prepaid label > Get prepaid label and QR code). 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 will be held for 14 days.
