There is no longer a link between Paypal and Ebay and you should be able to pay for the postage using other methods.
However, another member made the same complaint yesterday so I am assuming this is a glitch.
I would do two things.
Report it to Ebay as a technical issue.
Buy the postage independently. You don't have to buy your postage through Ebay and could take the parcel to the Post Office or buy a label direct from Parcelforce or via Royal Mail's click and drop website service.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)