You can either cancel/refund the transaction and relist with correct postage for the buyer to buy and pay again.
Or you could buy the correct postage from your own pocket. If you paid for the 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 automatically in 30 days.
Without tracking on ebay the payment will be released in 14 days.