You needed two separate cases and two separate parcels.
The cases would then stay open until it showed the item had been received by the seller and that they had refunded you.
It may be worth contacting Ebay Customer Services, explaining what has happened and seeing if they will help. Unfortunately, you have no proof that two pairs of shoes were returned to the seller.
Use the link to get a callback from Ebay.
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)