The "oversize" weight bracket (which allows you to choose and buy your own postage) is dependent on category and sub-category. Unfortunately ebay don't publish a list of which categories are eligible so it's a matter of trial and error during the listing process.
So during the listing process after selecting category and entering item title scroll down and click on the box under the heading Package Size. At the very bottom of the pop-up you're looking for:
Oversized
Above 20 kg or 61 x 46 x 46 cm
If it's there select that and you should get:
Domestic postage
Simple Delivery is unavailable for this item
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 the option doesn't show up try changing sub-category or category.
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 Simple Delivery 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 may be held for 14 days.
