But sometimes I have mildly inconveniencing experiences with the postal service in my extremely rural town that require me to navigate my extremely rural town's nearly non-existent public services so we should absolutely surrender complete control to Amazon

Milten Friedman is the reason we are where we are today.

For most of the rest of the world, that's about 219 mm × 137 mm × 41,3 mm

It's only in your head you feel left out or looked down on...

moving from Europe to America the amount of times I'm like "it's 12 3/8ths" to try to, yknow, join in, and everyone's like "call it 12 or 13"

motherfucker that's a huge gap!

Last package of the da... Yo wtf?!?

But do you fit into that box? 🤔

I'll go one better.
A (non-spinning uncharged) black hole with diameter 1+5/8th inches (so it fits in the box) has a mass of about 2.3 earths.

(Near as I can tell QGP filling the whole box is around a ten billionth of that.)

Of course the box would Very quickly no longer be outside the black hole. QGP would also cause the box to no longer be a container in short order. To put it mildly.

If mailing 70 lbs of unstable particles that can't exist outside of a lab is wrong, I don't wanna be right.

No you mean theoretical. As neutronium is a theoretical substance. To our knowledge there's no way to find it outside of neuron stars. It is therefore physically impossible, within our current state of knowledge.

It's highly unlikely, bordering on theoretically impossible to assume that mankind will be able to synthesize enough to fill a cardboard box with. Then the practical side says even if that was possible, there would probably no way a cardboard box could contain that (and a plethora of other practical impossibilities).

I guarantee that it is physically impossible to fill a cardboard box with pure neutronium. Is it physically possible to get over 70 lbs of the stuff in there in a stable, shippable manner? I don't know, and neither do you. It's certainly far, FAR beyond the capability of any technology on Earth, but I guess it might maybe possibly not break the laws of physics. I can't prove that though, and neither can you, so neither of us can actually prove the statement wrong.

Okay so I originally assumed this was probably due to some union rule or something like that. But I didn't find any reference to it in the NALC guidelines, anything in the USPS resources center (which is hard to use), anything in google searches, and the original employee documentation or spec.

I did find the USPS History section and it turns out they have someone whose job title is "Postal Historian", Stephen Kochersperger.

But, anyways, I found the address (not email of course haha) for the USPS history office so I have wrote up an letter and put it in the mailbox. I will eventually update yall

This is the case for most "Dumb laws": there's an outlier that becomes kinda silly, but it's not really worth the effort to change.

I saw one "It's illegal to hunt Blue Whales in Idaho". Because it's illegal to hunt endangered species in Idaho, and Blue Whales are endangered, not because legislators were super concerned about saving Idaho's whale population.

Such devices exist, namely stars. Neutron stars are theorized to have neutronium at their core, essentially a soup of neutrons so densely packed that nothing else fits between them - in order words, the densest theoretical material (osmium is the densest material found on Earth).

I believe that would be some form of fusion

The surface area of the box is about 135 inches. If this surface area were spread over a sphere, it would have a diameter of about 6.5 inches and a volume of nearly 150 cubic inches (nearly twice the volume of the uninflated box!). 150 cubic inches of osmium weighs about 120lbs.

So, indeed you could exceed the weight limit of the box by ballooning it out and filling it with something that's at least 7/12ths as dense as osmium (or a little more dense than lead).

Good news, after obtaining a piece of neutron star in those dimensions, you wouldn't need to worry about it anymore.