All military addresses should be formatted as outlined below and contain the APO/FPO/DPO designation as the "City", along with a two character "State" abbreviation of AE, AP or AA and the Zip Code.   

Orders with any of these addresses can only ship via USPS, FedEx SmartPost (which uses USPS for last mile), and UPS SurePost (which uses USPS for last mile).  

For example:

Army/Air Post Office (APO)

PFC JOHN DOE

PSC 3 BOX 4120

APO AE 09021

Fleet Post Office (FPO)

SEAMAN JOSEPH SMITH

UNIT 100100 BOX 4120

FPO AP 96691

Diplomatic Post Office (DPO)

JOHN ADAMS

UNIT 8400 BOX 0000

DPO AE 09498-0048

The City will always be one of the following:

APO (Air/Army Post Office)

FPO (Fleet Post Office)

DPO (Diplomatic Post Office)

The State will be one of the following:

AA (Armed Forces of the Americas, excluding Canada)

AE (Armed Forces of Europe, Middle East, Africa and Canada)

AP (Armed Forces of the Pacific)

Related Questions:

How do I format a military address?

Which carriers ship to military addresses?