The postal monopoly on first class mail, however, is not a “constitutionally legimate” function of the federal government. The creation of a monopoly for this insitutution is not mentioned, or even hinted at, a single time.
The monopoly on 1st class mail isn’t specifically mentioned, but the Section 8 enumerated powers of Congress include “To establish Post Offices and post Roads”. In that sense, the USPS is indeed constitutionally authorized. The exact function of the Post Offices established is not defined in any detail, but the current operation of the USPS has far more constitutional grounding than just about everything else the Federal government does outside of the DoD.