The weight limit is 70 pounds = 31.8 kg. Lead is 11.35 g/cm^3.

Small flat rate box, 21.9 x 13.7 x 4.12 cm = 1236 cm^3 = 14kg; you can fill 100% with lead and mail it. Tungsten is also allegedly fine, but it will weigh 23 kilos and be quite difficult to pick up (can't get a finger under an edge...)

Medium flat rate box = 95kg; you can fill it 33% with lead, or ~45% with steel and mail it.

Large flat rate box = 144kg; only 22% lead.

(CGS units give me nightmares, so I'll swap to SI.)

For solids at room temperature and pressure the best you could do seems to be osmium or iridium, unless you have access to heavy transactinides.

  mass/kg substance
  2.57e5  small flat rate box
  2.26e5  osmium 
  2.25e5  iridium
  2.65e5  meitnerium (theoretical)

Considering the expense of synthesizing meitnerium and the half-life which is measured in seconds, I would recommend getting insurance as well as express shipping if you do try.

Less than a second for all known isotopes I can find [0] So by the time you get to the post office it's under the weight limit (and you and everyone nearby are dead from the massive radiation dose probably)

[0] https://en.wikipedia.org/wiki/Meitnerium#Stability_and_half-...

What's wrong with 278Mt? That should give you a lesiurely 4.5 seconds.

The problem is I missed it wasn't ms like the rest of them. Still stands though you'll lose too much density by the time it's weighed even assuming you could mass create the Mt all at once.

Not with that attitude.

If you've got a particularly slothful postal worker you might consider topping up your meitnerium with a similarly sized sample of roentgenium-282. If you don't have any to hand then perhaps you'll have better luck producing tennessine-294 and waiting a couple of minutes. Amusingly your trip from Tennessee to x-ray land will take you via Moscow and Japan.

(Beware that you might end up making the box bulge a bit, since you'll necessarily go over the size limit by including less dense materials as well as necessary apparatus.)

I don't think it was a coincidence. Basically everyone who shipped raw metals in quantities that would reasonably fit in those boxes, did so.