Good info on this thread. I always like to mark my packages "CAUTION CAUTION PERISHABLE PLEASE NO HEAT OR COLD", "LIVE TROPICAL FISH", and "THIS SIDE UP", (with arrows). Here is section 526.6 of the USPS, Small, Harmless Cold-Blooded Animals. It refers to animals that can be mailed via the USPS. Also a link is provided for more in depth information on mailable animals and those that are not mailable.
526.6 Small, Harmless, Cold–Blooded Animals
Small, harmless, cold–blooded animals, except for snakes, turtles, and turtle eggs, are mailable only when they meet certain requirements. For some examples, see Exhibit 526.6.
Exhibit 526.6Requirements for Mailing Small, Harmless, Cold–Blooded Animals (Except Snakes, Turtles, and Turtle Eggs)
General Requirements:
All animals in this group:
Must be able to reach their destination in good condition in the normal transit time between the mailing and address points.
Must not require any food, water, or attention during transport.
Must not create sanitary problems.
Must not create obnoxious odors.
Specific Requirements by Animal:
Baby Alligators
Baby Caimans
Chameleons
Frogs
Lizards
Newts
Reptiles and Amphibians
Salamanders
Tadpoles and Toads
Animals must not exceed 20 inches in length.
No additional requirements, except the applicable standards in DMM 601.9.
Packaging must comply with DMM 601.9.3.11.
Specific Requirements by Animal:
Goldfish
Tropical Fish
Fish must be held in a securely sealed primary receptacle.
Primary receptacle must be cushioned with sufficient absorbent material to take up all liquid in case of leakage.
Primary receptacle and absorbent cushioning material must be sealed within waterproof outer (shipping) packaging.
LINK TO MAILABLE ANIMALS SECTION
LINK TO NON-MAILABLE ANIMALS SECTION