You could cancel/refund the transaction with the buyer's agreement (via the dropdown actions menu arrow for the item on your orders list) and relist for the buyer to buy and pay again.
Or you could buy and pay for the postage yourself. To add that tracking number you still need to generate the original prepaid label. Then change the tracking via View Order Details > Postage > Track Package > dropdown arrow > Add Tracking. That needs to be done on a desktop browser.
Afterwards if you paid for the original label you can cancel it here:
https://www.ebay.co.uk/claims/simple-delivery/refund/seller
The item has to be sent with a tracked service for you to receive the refund for the unused SD label. If the buyer paid for it it will be refunded to them automatically in 30 days.
@micfr-6155
