Under Simple delivery if you have entered the correct weight and size of the item, if there was any discrepancy in postage price, eBay would pay that difference. This would not be the case if incorrect details were entered.
Contact eBay Customer Services they may be able to see what has happened here.
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.
@zebra_leasing