Did you buy the postage label from eBay? You can void that unused label, and receive a refund. Then, either buy another correct one through eBay, or take to the Post Office and they will charge you for the correct label.
If you buy over the counter postage you will have to manually mark the item as dispatched and you may need to contact eBay Customer Services for them to correct the tracking number for you.
To receive a refund for the unused label, it may take 24 hours for it to appear, 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.
If you need to,
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.
@jeverden