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.
The Sedition Act clearly violated individual protections under the first amendment of the Constitution; however, the practice of  judicial review , whereby the Supreme Court considers the constitutionality of laws was not yet well developed. Furthermore, the justices were all strong Federalists.