might sound like a silly question, but did you post to the address that appeared on the eBay "Your item" sold notification OR the address on the PayPal payment notification?  The reason I ask is that there have been some posters in recent times who have posted to the eBay address and had no coverage because the PayPal address was different to the eBay address.

Secondly, check the buyers feedback and see if there are any comments there that might indicate that this is 'pattern behaviour' by this buyer ID.  If there is a pattern, that would certainly bolster any defence against a chargeback.

Thirdly, put that eBay ID onto your Blocked Bidder List.

Fourthly, please do come back and update this thread with information as this case proceeds as that information will help others who find themselves in the same/similar situation.