Interesting, are you sure about this? The only clause I find in the constitution mentions post offices and post roads (perhaps that's what you're thinking of?), but nothing about their security. To my knowledge federal agencies were only established by the Administrative Procedures Act after WWII (which is basically the "constitution" of federal agencies), but I'm not sure if there's other similar terminology that might be in the constitution. If someone has a link to the clause that would be interesting.

Yeah buddy. The security service is just like internal affairs for the USPS. It's actually pretty creepy inside the USPS they enter through different doors, they have catwalks with two-way mirrors, etc. The SSF and DCF or whatever they're called (been a while) are super creepy to observe. You are CONSTANTLY being watch when you work there, but unseen faces. There is a reason 'going postal' became a thing. Imagine that atmosphere!

Having worked there, you drown out the catwalks and just consider them like fancy camera monitoring. We never, ever saw law enforcement, so it was out of sight, out of mind.

Census is in the Constitution as well

Census as an activity is, though not a Census Department specifically. Though that may be a quibbling distinction.

Armed forces would be another Constitutionally mentioned function.

Pretty the sure the Census is mostly conducted by the USPS though.

Incorrect.

Department of Commerce.

Ah, good to know, and thank you. Albeit, logistically carried out in large part through the mail. Would it be safe to say that they are intertwined, and the Department of Commerce is another made up beaurocracy, and if nixed the USPS would conduct the census?