With Simple Delivery the buyer pays for the postage.
However, did you originally buy the postage through eBay ( Packlink), but didn't use it? Then, you can get a refund for that unused label, you may need to wait 48 hours for it to appear before being able to void 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.
If the service and/ or tracking is now different to that marked when dispatched, then you do have to contact eBay Customer Services to do this for you, but do so only very first thing in the morning when the lines first open at 8 a.m. as there's more chance of Dublin answering.
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
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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.
As said, I recommend contacting CS first thing in the morning as there's more chance of Dublin answering.