It depends.
If it was Ebay Simple Delivery, and you paid for the postage, you should get a refund from Ebay but that may take up to 30 days. You don't need to do anything in this case.
If it was a business seller, or they didn't use Simple Delivery, you would have to ask the seller for a refund.
"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)
