This question comes up fairly often on the firearms forums and often you will receive a number of well-meaning responses, some right and some wrong. The best resources are the ATF and USPS websites so you can reference your actions in writing. Do not depend on the USPS personnel knowing the regulations.
Look up www.atf.gov/firearms/faq/faq2.htm
for the ATF answer to your question. Scroll down to B-7 and the answer is clear.
For the USPS regs look at web site www.net-ffl.com/pub52_43.html
. They tell you if and how you can mail handguns and longuns. Good luck.
Merry Christmas- oldandslow