If the claim is for unauthorised use as opposed to an item not received or not as described then it is always worth contacting the buyer and demanding payment  or you will report them to the police for obtaining goods by deception or fraud depending on if they have wrongly claimed somebody else used their card or they have used another person's card. Of course with no proof of delivery it would only be a bluff but if you have tracking even though it is not up to Paypal standard you may scare them into paying for both the goods and any charge which is made by the bank and passed on by Paypal.