If you have not paid to get the item, then you can open an 'item not received' case through Ebay and get a full refund.
Royal Mail will send the item back to the seller if the package has a return address on it.
This is not fraud - the seller has made a mistake with the postage. Please don't contact the police as this is not a police matter.
"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)