Theyre also a separate entity not the federal government.
You are the lawyer but they are covered by Federal Claims Tort Act.
As far as not being part of the federal government.
The Supreme Court has even weighed in on what being “independent” means for the USPS, in an opinion from 2004:
The PRA’s designation of the Postal Service as an “independent establishment of the executive branch of the Government of the United States,” 39 U. S. C. §201, is not consistent with the idea that the Postal Service is an entity existing outside the Government. Indeed, the designation indicates just the contrary. The PRA gives the Postal Service a high degree of independence from other Government offices, but it remains part of the Government.