James Madison wrote:
http://constitutionus.com/Try reading this. Article 1 Section 8. The Post office is constitutionally mandated. Find me libraries anywhere in the Constitution. Don't give me the "general welfare " crap either. Enumerated powers of the federal government.
Libraries, charging stations are fine if your state wants to pay for them.
The post office is constitutionally permissible. It is not mandated. If congress wanted to do away with it they could. In my opinion they should.