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.
Citizens protesting a nuclear plant in Tamil Nadu, as you read this, have been charged with sedition. The Sedition Files: How an Indian Cartoonist Becomes a Criminal Dilip DâSouza September 20, 2012. I refer to the Alien and sedition Acts, signed into law by President John Adams in 1798.