since u r in usa contact local paypal office , or contact the buyer who actually picked up the package then talk with them physicaly not by phone or by net ,if not resolved go small causes court and file a claim case with all proof and remember u must party the buyer as a first and add paypal as a second party (proforma defendant) If the buyer really picked up the package from you and u have proof then dont worry U will win the case and according your negative balance will be adjusted . thanks
