You can still recieve model rocket engines by mail.
U.S. Department of Transportation (49 CFR), require that (model rocket motors contain no more than 62.5 grams of propellant per device) and are properly packaged, marked, labeled and documented in accordance with this regulation.
UPS, FedEx and other carriers continue to carry these items. The United States Postal Service also will ship Toy Propellant Devices; (Model Rocket Motors and Igniters that are pre-approved for mailing by the USPS) that contain no more than 30 grams of propellant per device.
The Homeland Security Act includes the "Safe Explosives Act". This has placed a greater responsibility on the Bureau of Alcohol, Tobacco and Firearms in an effort to keep explosives out of the hands of terrorists. Therefore, there are now more explosives regulations.
Visit http://www.atf.treas.gov/explarson/safexpact/modelrockets.htm to obtain accurate information with regard to the ATF and model rocketry.