Do we have a source for this or are you just joking?

Doesn't matter. You won't see it til it too late.

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

What is the densest substance we can fill the box with?

Hmm, that might make it feasible to do with something that you can actually buy in large quantities, like tungsten! Would still probably cost four or five figures though.

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.

The common popsci factoid tells us that a teaspoon of a neutron star weights as much as Mount Everest, so maybe.

Good news, it's 20-30 years away!

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