US Mail with a delivery destination to a military address outside the continental US can only be shipped via the US Postal service and must be shipped with a special label/custom form.
APO refers to "Army Post Office", while FPO refers to "Fleet Post Office". The Ship To "Country" on the address will be United States, with the following state abbreviations: AE (Armed Forces Europe), AA (Armed Forces Americas), and AP (Armed Forces Pacific).
ShipBridge can print out the special APO labels with the accompanying customs form. A label configuration must be done for APO labels. Under the label printer configuration for USPS - Endicia or Express One - add the USPS APO/FPO service and configure with a PDF printer. Read about configuring labels here.