Is there a special rate of U.S. postage for an APO address?

No. standard postage applies as if you where sending a letter or package to another adderss in the US, however if you need to send a package you will need to fill out a customs form you can get from the post office. for more info go to the USPS website at www.usps.com/supportingourtroops/supportingfaqs.htm