I was right. USPS shipping regs here:
http://www.usps.com/cpim/ftp/pubs/pub52.pdf

521 General Requirements
The full cooperation of the mailer is essential in order to safely and effectively
transport animals through the mail. The following factors are normally applied
to all shipments of mailable live or dead animals:
a. Protection of postal employees and the public against harm from
dangerous or diseased animals.
b. Protection of the mail and the environment against the following:
(1) Damage to the shipping container or other mailpieces from either
the animal or the refrigerant used (i.e., moisture or condensation
from melting ice, or heat from dry ice).
(2) Obnoxious odors and noise.
c. Protection of animals against death, or protection of animal specimens
against spoilage, taking into account the following:
(1) Expected time in transit.
(2) Expected temperature in transit (weather conditions).
(3) Packaging, including insulation against impact, heat, cold, and
suffocation.
d. Ability of an animal to survive without food or water during transport.
Live animals must be transported without food or water because
liquids, moisture, and loose foodstuffs can cause damage to the
shipping container, other mail, and postal equipment during transport.

(And here's the part about reptiles: )
525.3 Reptiles
All snakes, turtles, and poisonous reptiles are nonmailable.

Live scorpions can be shipped. See sec 526.1.d.