Did you then buy over the counter postage at the Post Office? That's your best option here for this item, if Simple Delivery was selected then a refund is not made for the unused label.
If you cannot manually change the tracking where you mark the item as dispatched, you should be able to using a PC / laptop and Chrome then contact eBay Customer Services they will do this for you.
This is the easiest and quickest 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 8 a.m. - 10 p.m. on weekdays
9 a.m. - 6 p.m. on weekends.
Automated agents will be available on chat outside of the above hours.
I recommend contacting CS first thing in the morning as there's more chance of Dublin answering.
@raved-86