I recently mailed a package using a Pitney Bowes postage meter. After calculating the postage there is a screen listing restrictions of items which can not be mailed through the US postal system. I suspect most people just click through without reading it, which is a shame. It is a fascinating read, and is reproduced here for your edification and bemusement.
This is great stuff. I have several observations.
Note the "infernal machines" phrase in section (d). What is an infernal machine? The Pitney-Bowes restrictions appear to come directly from the US Postal Service Domestic Mail Manual C021, but the phrase is not subsequently defined there. Is it something like the puzzle boxes from Hellraiser? I can see why we might not want those to be spread around...
I had no idea the scorpion industry lobby was so powerful. In fact, I had no idea that there was a scorpion industry nor that they had a lobbyist, but they scored their own exception in section (b). Poisonous snakes need not apply, only scorpions can be mailed for medical research purposes.
In the final paragraph it explains that you're not allowed to ship explosives under any circumstances. Also, you need to ensure that containers which you previously used to ship dynamite have been cleaned of any residue. I'm sure how you could possibly have containers which were previously used to ship dynamite, if you're never allowed to ship dynamite.
Finally, in addition to the rules against shipping materials which could kill or maim note that you're not allowed to ship anything smelly or stinky. That would be gross.