Ok,  I just thought of this after the fact but it may help me (and other Acaeum members) in the future.

 PAYPAL only lets you challenge a few transactions in any given period of time.  I think it's 3 per year but I don't remember exactly.

 If someone tells you that they did NOT receive your shipment DO NOT refund their payment.  Let them file a dispute through PAYPAL.  Do so politely and let them know that you're not questioning the validity of their claim but that such claims need to be filed in the proper way through PAYPAL to avoid fraud.

 Basically, this way we are limiting scam artists to (if memory serves me) 3 rip-offs per year or however many PAYPAL will tolerate.  I know it's not an open ended option that can be used over and over again...

 So, if someone wants a refund because they didn't receive your shipment; make them file for a refund through PAYPAL.  That way a record is created and PAYPAL can take notice if such claims are habitual.