If you used Special Delivery for this item, then the buyer has paid for the label, and if not used, they will receive the refund but it can take 30 days, may be best to send them a message to explain this.
Be aware, if you then use your own label and tracking, this may not update as delivered, and you may have to wait 14 days for the payout to be released, if the tracking cannot be edited.
If you purchased the label under Custom Postage, this is how to receive a refund for the unused label.
Find your label with this link . https://www.ebay.co.uk/ship/lmng/labels-history/seller-hub and to the left of the sales record there is a drop down menu under 'Actions' click on that , then click on 'Void Label'.
The refund will take between 5 - 21 days to process.
@nelly669