If you don't return the item within the timeframe, the case will close in the seller's favour.
Personally, I would send it myself using a tracked service.
Then, contact Ebay Customer Services. Explain that you have used your own postage and get them to add the tracking number to the case. (Unless you can add the tracking to the case yourself.)
Use the link to get a callback.
https://www.ebay.co.uk/help/eua?id=5190
"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)
