Actually, libertarians would take issue with the enumerated power of congress to establish post offices which has nothing to do with protecting them from any violation of their "negative rights."
No they would not. The Constitution is not a libertarian document. It is the document which defines and empowers the US govm't. It says what it says.
Now if you're refering to judging that document on libertarian grounds alone, you'd be correct. A postal service is not a legitimate power of govm't under libertarian theory. It is properly private enterprise. The reality is though, that any such discussion is of no legal validity.
"which has nothing to do with protecting them from any violation of their "negative rights."
Rights don't have polarity, see #124.
Accept that the Post Office is a government monopoly supported in part by tax money. Taxes are theft.
Also the recent proliferation of private carriers points out that there are much better ways, cheaper too, to move the mail.