I have taken to video documenting everything, and using tamper proof tape. One continuous shot, etc. Signature confirmation. And USPS proof of acceptance, and a photo of it at the PO. That way if the buyer accuses me of not sending the item, I have more than the average proof. Excessive and paranoid on my part...yes.
Based on you receiving empty box, sounds like it is time to start video documenting the receipt of items.
I would hope, PP would consider such evidence in a claim.