ebay have said if you purchase your own label to add that tracking number you still need to generate the prepaid label and then go to the More Actions dropdown menu to add the new tracking (needs to be done on a desktop browser).
After that you should be able to cancel the original label here:
https://www.ebay.co.uk/claims/simple-delivery/refund/seller
Or if the buyer paid for it it will be refunded to them automatically in 30 days.
Even if you cannot manage to do the above the payment will be released anyway in 14 days.
