Plus they aren't really Federal anymore. Remember when the Postal Service "privatized"
"In 1971 the Post Office Department was reorganized and became the United States Post Office. It was an independent establishment of the executive branch of the Government of the United States. The mission of the Postal Service remained the same, as stated in Title 39 of the U.S. Code:"
They aren't fully private and they aren't fully Government
https://federaljobs.net/blog/working-for-the-united-states-postal-service-usps-part-1/