SD offers buyers the cheapest postage options so that does not include first class.
Your options are to use free postage but buy your own postage. It gets a bit fiddly after that. To get the buyer's address and mark dispatched you have to open the original prepaid label (Get prepaid label > Get prepaid label and QR code). You can't change the tracking but 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.
The other alternative would be to register as a business seller then you can use whatever postage you like.
