Are you still abale to oost the item with the correct label? It might give you a damaging efect on your account if you don't.
Here's how to cancel the label and receive a refund, you may need to wait 24 hours for the label 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.
Then just take the re packed item to your Post Office and buy over the counter postage. You will then need to mark the item as dispatched manually , and if you cannot change the tracking number manually, contact Customer Service, they'll do this for you.
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.
@jswc7794