In Article I, Section 8, it says that the congress shall have the power,
"To coin Money, regulate the Value thereof, and of foreign Coin, and fix the Standard of Weights and Measures;"
and
"To establish Post Offices and Post Roads;"
Further along, it says,
"No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money;"
and then it says,
"No State shall, without the Consent of Congress, lay any duty of Tonnage, keep Troops, or Ships of War in time of Peace, enter into any Agreement or Compact with another State. . ."
I suppose that a state could create its own post office as long as it delivered the mail only within that state, or that a state could create its own post office in the event of an enemy invasion. And I suppose that several states could obtain permission from congress to enter into a compact to deliver mail among the states in the compact. It would be helpful if the legislators in those states were immune to criticism in the form of torchlight marches and hanging people in effigy. I'm surprised that New York hasn't tried it, the unions could add another 100,000 members to bloat themselves at the public trough.
"This Constitution, and the Laws of the United States . . .
shall be the supreme Law of the Land;
and the Judges in every State shall be bound thereby,
any Thing in the Constitution or Laws of any State to the Contrary notwithstanding."
How could this be more clear?
If true, the courts ignored it for 150 years. They're still ignoring it on some amendments, making it 220 years now.
Now, you can either believe there was some massive conspiracy by the courts spanning 150 years or you can admit that your assumption about the "clear" meaning of the Supremacy Clause was wrong.
How much longer, how many more posts, are you willing to make yourself look foolish? It's up to you.