You made a mistake, easy to make, but you closed the return and the rules are very clear, only one case can be opened per item, so another for the return cannot be made.
I know you've contacted Customer Services, but you do get different responses from different agents. It is worth trying again, but I strongly recommend contacting CS at 8 a.m. on weekdays, as there's more chance of Dublin answering. Only speak to them, you'll hear if it's the dulcet tones from the Emerald Isles, as it's a Call Back system.
Explain what was done in error, and ask if they would kindly reopen the case and provide the correct return postage as the original didn't cover the cost for the items.
Contact them through this link here.....
https://www.ebay.co.uk/help/eua?id=5190
@sindy_7