The history of the US Post Office is a case study in the Federal government turning the Constitution on its head.
The "post Roads" provision was voted into the Constitution on Sept. 14, 1787 to defend free speech and to assure that if no other means were available to deliver letters, the Federal government could tax to build roads and post offices adequate for delivering those letters in defense of free speech.
In practice, the US Post Office became a commercial monopoly using the police powers of government to limit freedom of speech by forbidding other means of delivering letters. It was a system of political patronage instead of a defender of liberty.
The intent of the Sept. 14, 1787 vote in the Constitutional Convention was to defend free speech, not to create a government monopoly that would restrict the flow of speech.