Ideally 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 probably need to opt out of Simple Delivery to choose your own postage. The opt-out for large items is not working the moment but see the workaround solution on here:
https://community.ebay.co.uk/t5/Seller-Central/how-do-you-list-anything-bigger-than-61cm/m-p/7882486...
The alternative would be to pay for the correct postage yourself. 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.
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.
