Here is an excerpt form the Domestic Mail Manual from the USPS. I think "d." covers your question. If you want to read further you can go to usps.com or to any post office and look in Chapter 6. Hope this helps you. Remember not to blame the USPS or employees as we are governed with oversight by Congress, who is directly responsible for the current rate increase.
8.5 Harmful Matter—General
Except as provided in this document, any article, composition, or material is nonmailable if it can kill or injure another or injure the mail or other property. Harmful matter includes, but is not limited to:
a. All types and classes of poisons, including controlled substances.
b. All poisonous animals except scorpions mailed for medical research purposes or for the manufacture of antivenom; all poisonous insects; all poisonous reptiles; and all types of snakes, turtles, and spiders.
c. All disease germs or scabs.
d. All explosives, flammable material, infernal machines, and mechanical, chemical, or other devices or compositions that may ignite or explode.