Did you get it back? If so you could cancel/refund the transaction with the buyer's agreement (via the dropdown menu arrow for the item on your orders list) and relist with correct postage for the buyer to buy and pay again.
Or buy and pay for the correct postage yourself. You cannot add a new tracking number, all you can do is message the buyer with the new tracking details. You would still be able to add the new number to a "Not received" case if the buyer opened one though.
If the buyer paid for the unused label it will be automatically refunded to them in 30 days. If you paid for the original label (ie. have free postage on the listing) you can cancel it here:
https://www.ebay.co.uk/claims/simple-delivery/refund/seller
If you didn't get it back and they are sending it to the buyer with insufficient postage warn the buyer about it. You will have to send them the cost to cover then underpayment but do that after they pay for it, in case they don't have to.
