We don't have a choice unless the items are exempt from SD (£10/100g items, letter or large letter in some categories, or extra large/heavy items).
If you use free postage (seller pays) you can cancel unused labels here:
https://www.ebay.co.uk/claims/simple-delivery/refund/seller
but to get a refund you have to add the tracking into the system via View Order Details > Postage > Track Package > dropdown arrow > Add Tracking.
If buyer paid for postage they get a refund automatically in 30 days.