Why will a post office not accept the item? What is it? Is it something that cannot be posted, then you'll have to contact eBay Customer Services.
If the label you've been sent doesn't cover the weight of the item, again you need to conatct CS and they'll provide the correct postage label and charge it to the seller's account.
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.
@ruwat_13