When shipping to a US military address, please make sure you enter APO or FPO as the City, along with the appropriate State Code for the location (AP, AE, AA, etc.) and the correct zip code. An example would look like this:
PFC JOHN DOE
PSC 3 BOX 4120
APO AE 09021
For more information about shipping to US Military bases, please see the USPS Military page.