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 with correct postage for the buyer to buy and pay again. You will still be using ebay's Simple Delivery and confined to their postage choices.
If the item is large/heavy you can relist and opt out of SD. See solution on here:
https://community.ebay.co.uk/t5/Seller-Central/how-do-you-list-anything-bigger-than-61cm/m-p/7882486...
Or you could buy and pay for a different postage yourself. To add that tracking number you still need to generate the original prepaid label. Then change the tracking via the More Actions dropdown menu or View Order Details > Postage > Track Package > dropdown arrow. 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
If the buyer paid for it it will be refunded to them automatically in 30 days.
