its much easier to buy them in the USA, like guns, bullets, grenades. to damage an airplane you dont need much: just a mix of molotov cocktail, and aluminum and metal shavings a-la Walter White in order to penetrate and ignite the fuel tank of a strategic nuclear bomber.

Why to use explosives at all? Thermite compound could be easily bought online and should be awesome for such fragile taget as a plane.

Have you seen a footage of "fire dragon" drones?

No need to buy compound and trigger authorities. Thermite is just metal oxide powder and aluminum powder that can be made at home, just add magnesium from matches to ignite and thats it.

Who needs explosives? Spring loaded pointy rod to the skull or razor to the neck

Or boxes of chocolates?

https://www.youtube.com/watch?v=Dy6uLfermPU

Spring Surprise??!

I doubt the post office does that much screening.

After Kaczynski? And we're talking about international parcels here. Even if the post doesn't bother to screen customs certainly does.