For a refund for that unused label, find your label with this link . You may need to wait 24 hours for the label to appear. 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.
Buy another label at the Post Office if you wish , but if you cannot manually change the tracking you may need to contact eBay Customer Services provide that number and they'll do this for you. Buying at the PO you will also need to mark the item as dispatched manually.
This is the easiest and quicket way to contact eBay Customer Services, for a Call Back option.
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
Lines open between 8 a.m. to 10 p.m. on weekdays.
9 a.m. to 6 p.m. on weekends.
I recommend contacting CS very first thing in the morning, when the lines first open, as there's more chance of Dublin answering.
@dwyster