SD labels cannot be cancelled but if they are not used the cost is refunded to the buyer. Any additional postage paid for by you is not recoverable.
The correct thing to do would have been to cancel the transaction and relist with the correct parcel weight on the listing so that the postage was sufficient. Always a good idea to have the item fully packaged first so you can put correct details into the listing process.
You can contact ebay to get the new tracking details on the order so that it shows as dispatched. You can use callback on here:
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid