USPS..Who are you??

I have been hearing for some time that the USPS is not a government
agency. A seperate entiety but sanctioned by the US Governmentis my
thinking.
A local diamond merchant sent a package via US Mail and insured it well
enough to cover the contents. USPS somehow "lost" the package and
refused to pay the insurance.
Dealer tried to sue the USPS but was told they has "sovereign immunity"
and therefore he could not sue.
How does a non-government agency claim sovereign immunity? Any lawyers
out there? (Or do I have the wrong conception?)

Ralphael, the OLD one.