Feedback doesn't help with this new system unless it's an item of £10 or less in value.
Use a laptop / PC and try Chrome, see if you can manually change that tracking number where you marked the items as dispatched.
If not contact eBay Customer Services, they'll change that number for you. Their lines open shortly at 8 a.m.
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 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.
If you purchased a postage label from eBay and have not used it this is how to receive a refund. You may need to wait 24 hours for it to appear before being able to cancel it.
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.
@webb3737
