Sometimes it is a simple error and the bank has done a chargeback without the buyer's knowledge so email them and tell them what has happened and ask them to either sort it out and cancell the chargeback or remake the payment into your bank account (give details).

If you don't hear back from them then I would call them, make it very clear that if the chargeback goes through you will be reporting them to the police for obtaining money by deception. Of course as you have no proof that anything has actually been delivered to the buyer (they could just claim they left feedback for the wrong item) this is a bit of an empty threat.

If you fight the chargeback and lose, which you will as you do not qualify for Paypal seller protection, it will cost you an extra $15!

This can be avoided by using click & send which is cheaper than regular parcel post or red satchels anyway and covers you for up to $100 with AP if you use the signature required option. For anything over $100 you should always use registered post with extra cover.

Other than that the only other domestic services that qualify for Paypal seller protection are Platinum Express, ordinary express (not the satchels) and eparcel.