It's called a Military Care Kit or Mili-Kit. From the USPS website:
The U.S. Postal Service has also introduced a Military Care Kit, or “Mili-kit” to make it easier for military families and friends to send care packages to their loved ones stationed overseas. Since the inception of the Military Care Kit, the U.S. Postal Service has shipped more than 150,000 kits. The kit contains three flat rate boxes, two cube boxes, nine address labels, and 16 customs forms. This kit may be ordered by calling the USPS Expedited Package Supply Center at 1-800-610-8734. There is no charge for the kit.
Here's a link to this info on the USPS website:
http://www.usps.com/communications/newsroom/localnews/in/in_2007_1004.htm
It's been a while since I got one of these, but when I last ordered one not all of the boxes were Flat Rate, but they were all Priority.
Hope this helps! 
"No person was ever honored for what he received. Honor has been the reward for what he gave." - Calvin Coolidge