APO, or Army Post Office, and FPO, or Fleet Post Office, are forms of address used by the United States military to deliver letters and packages to service personnel. APO addresses are used by both the Army and Air Force, while FPO addresses are appropriate for anyone serving in the Navy. Your mail will be charged at the domestic rate, but be aware that restrictions exist on what can be sent to an APO or FPO address. You’re likely to have to complete a customs form for any packages.
Addressing an Item
When sending a letter or package to an APO or FPO, use the correct address format. This should start with the addressee's rank and name, followed by the full name of their unit; the Army gives this example: "123rd ENG 2nd PLT - B CO." The final element of the address is APO or FPO, followed by a ZIP code, which -- again using the Army's example, will look like "AE 09398-9998." Even if you know that a person is serving in a specific country, do not write that country’s name as part of the address as you risk the package being accidentally diverted into international mail rather than into the Military Post Office system.
Sending an Item
Items can be sent to APO and FPO addresses only via USPS, and you must address it to a specific member of the military -- it is no longer possible to send items to, for example, “Any Service Member.”