Of course the buyer received the item, that is not under dispute. The card holder has reported the transaction as a fraudulent use of their card something Paypal will protect you against if you can prove to their standards that you posted the parcel to the Paypal designated address.

You are on a hiding to nowhere disputing the chargeback with Paypal, much better to just tell them to go ahead and issue the refund,  If you contest the chargeback and insist Paypal fight it the bank will almost certainly win and Paypal will add a $15 charge to the amount you have to pay back.

You can of course go after the person who received the item and threaten to report them for fraud if they do not make immediate payment but with no proof of posting or delivery it would all be bluff.