We the People empowered the legislature to establish post offices via Article I Section 8 where it says, “The Congress shall have power to establish post offices”. We did not give them the power to abolish the United States Post Office Department, incorporate and sell under the name United States Postal Service but they did and never-the-less they must obey the law.
The last lawful legislation that still stands is the 1863 Act of the Thirty-Seventh Congress whereas the postal rate on all letters are 3₵ per ½ Ounce. 18 USC CHAP 83 §17263 declares it fraud to demands any rate of postage other than is provided by law punishable by fine or imprisoned not more than six months, or both. For history and more information read Postal Monopoly Laws.