APO and FPO work somthing like this:
you send it via USPS
USPS sends it to an army or navy base
at that point, they pass it off to the army/navy who then transports it to whereever the army guy is or to the ship the navy guy is on
so if you send something to a guy on an aircraft carrier in the persian gulf, the package may go to a FPO in florida , then the navy may take it on a cargo plane to some navy base near the gulf, then fly it out to the ship and deliver it onboard to the guy
this way no matter where a military person is deployed, they can get US mail at reasonable rates. if you shipped it to them to a local address (which they don't have anyway), you would pay normal international shipping rates and it would cost too much, so nobody would get mail while deployed
and navy people wouldn't get anything since you can't send stuff to the middle of the ocean any other way