You weren’t being asked to pay the original postage cost. The seller chose to pay that and didn’t charge you that amount - at least, not separately. The seller absorbed that as part of the total amount that you paid.

When it comes to returning an item as a CoM, you pay that return amount. Paying for the return postage label through eBay simply means that that cost is deducted from the refund, so that you gave no upfront postage cost. It’s nothing to do with the original postage amount.

 Unfortunately, if you open and then close a return request, it cannot be reopened.

If the seller is a business with a CoM return policy, you should be able to sort it out directly with the seller.

Or… you could try contacting eBay Live Chat during business hours and trying to get hold of a CS rep rather than a bot… and asking if they could reopen the return case for you as you didn’t understand that the case would be permanently closed. It’s a long shot but you won’t lose anything by trying.