The USPS often is mischaracterized as a quasi governmental or private entity. It is neither. The USPS is a government agency that was created by Congress to achieve various public purposes. Federal law defines what products and services the Postal Service may offer.
Sedition and seditious libel were criminal offences under English common law, and are still criminal offences in Canada.
Ironically, a sedition law used by the British colonial government to suppress nationalist dissent in the subcontinent during the 19th and early 20th centuries is being applied today to curtail what some critics feel are reasonable criticisms of the government.