If you know for a fact it was there, then call their bluff. Tell them you checked it and it is there. If they try to do a chargeback dispute it and claim that they received what they ordered. The merchant will win in this situation if the delivery has been properly documented. It would then be up to the customer to pursue the matter legally if they felt they had enough evidence to raise doubt, such as objective witnessess present upon openeing the box, weight discrepancies etc.