Is it legal to send charcoal briquettes through the mail in the United
States? Are there any regulations about the packaging or contents
being certified or the like?

Hello and thank you for your question.

Flammable solids are prohibited in international mail. Flammable
solids are prohibited in domestic mail via air transportation. A
flammable solid that can qualify as an ORM-D material is permitted in
domestic mail via surface transportation if the material is contained
in a secure primary receptacle having a weight of 1 pound or less; the
primary receptacle(s) is packed in a strong outer shipping container
with a total weight of 25 pounds or less per mailpiece; and each
mailpiece is plainly and durably marked on the address side with
?Surface Only? or ?Surface Mail Only? and ?ORM-D? immediately
following or below the proper shipping name.
http://pe.usps.com/Archive/HTML/DMMArchive1209/C023.htm#Rap29357

ORM-D (Other Regulated Material) material is a limited quantity of a
hazardous material that presents a limited hazard during
transportation due to its form, quantity, and packaging. In almost all
instances, the proper shipping name for an ORM-D material is consumer
commodity. Not all hazardous material permitted to be shipped as a
limited quantity can qualify as an ORM-D material. ORM-D materials
having the proper shipping name of "consumer commodity" are mailable
subject to USPS quantity and packaging standards.
http://pe.usps.com/Archive/HTML/DMMArchive0108/601.htm#wp1064962

Properly packaged charcoal will qualify as ORM material, and so can be
mailed via ground.
http://www.firefox-fx.com/ChemC.htm

Search terms used:
flammable prohibited site:usps.com
ORM charcoal 

Thanks again for letting us help.

Google Answers Researcher
Richard-ga

A very fast and thorough response, that took a few hours. The links were valuable.

I wonder how flammable styrofoam used for packaging is?  If untreated,
it is flammable and gives off black smoke.
It seems like books would be as flammable as charcoal or brickets, but
I had no luck finding flash points.

Not to sound to uhm smarta.. flashpoint of paper is Fahrenheit 451 :)

And that for charcoal or brickets?  I couldn't find their flashpoints.

It depends a little on the composition of th charcoal and if it
already contains a form of lighting fluid , if it doesn't the figures
are as follows

Spontaneous air ignition in powdery form may start at 315 degrees centigrade
Flashpoint in excess of 500 degrees centigrade
longterm exposure to a heatsource of minimum 260 degrees centigrade
will cause charcoal to ignite

hope this helps

Yes, thanks, that is rather what I expected.  Brickets have about the
same or a higher flashpoint than paper, ie, treating them as hazardous
material is a bit illogical (excluding those loaded with a lighting
substance).  And by the time they had been exposed to 260°F for a
longer period, there would obviously already be a pretty good fire.
Thanks again, Myoarin