Ebay would not repay the overpaid postage unless:
You are buying from a private seller
They are using Simple Delivery
They send all your items together so there are several unused labels.
(If it is a private seller using SD, they can't send you an invoice. Can you give one of the item numbers?)
Then, Ebay should reimburse you for the unused labels but it can take quite a long time.
You should really have checked with the seller before bidding/buying that they could offer combined postage.
"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)