It is fairly simple to sort this out.
Ask the buyer to close the return.
You will only be charged for the label if it is used.
Ebay Customer Services is available for most of the day - although they are often busy. Currently, for example, there is a 31 minute wait for a callback.
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
"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)