depending on where you are, if your in the same country its easy to proof otherwise. Make sure you have a receipt of documentation of the shipment that has the product's weight or weight of the package. Go to the manufacturer of the item and find out the weight. Submit your paperwork stating that the weight of the item must be a minimal weight of the product's weight because you have a document (the shipping receipt) that says the package was x amount in weight. If he tries to claim something else, then you basically just caught him in a lie. You can prove fraud right there, his own words was empty, mysteriously out of no were there's something in the box? Its just a statement the theif is going to use to change his story because he got caught commiting fraud.