I called up the USPS and got some insight on this. Incidently, the APO I'm talking about happens to be in the Middle East.

APOs are only useful if something is posted inside the US or from an APO/FPO address. If posted from within the US and the envelope is under 16 ounces, it's handled just like regular mail send from within the US to within the US. If it's a parcel or something that weighs 16 ounces or more, it's got to have a customs declaration and be given by hand to a postal clerk.

If someone is posting from outside the US (and not from an APO/FPO), then they include the country and city along with the APO address. The mail goes through customs of the country it's being sent to and is delivered by that country's postal service to the APO address.

It looks like if I want to get something from an IOP and have it shipped to my APO, it needs to go to my UPS Store, which has for years consolidated mail and sent it to me via USPS or courier. If under 16 ounces, it's go as regular mail. If over 16 ounces, it'll have a customs declaration of "letters" or something like that.

Will the mail/packages get more scrutiny? I'm told they'll be looking for things like letter bombs and anthrax if it's under 16 ounces.