Seems to me it gives them the power, if they choose to exercise it, to establish a post office. I don’t see that it mandates them to do it.
“To establish Post Offices and post Roads;”
How many post roads are there?
Very few so named; the most famous named one in the country is the Boston Post Road (aka US 1 nowadays, which is the "lower" post road along the shore line).
How many post roads are there?