I was very aware of possible problems with handling of Priority Mail packages. This is why I secured my parcels with "Gorilla" Duct tape. My boxes were nearly "iron clad" with all the tape I put on them and almost impossible to break open - no matter what. If you ship valuables in the future - I would suggest getting a roll of "Gorilla" tape and really sealing up the boxes on ALL edges like I did to mine.
Once again, sorry to hear about your problem and I hope you get a complete settlement.
Take care,
Jerry