army post office
NPO is navy post office
it is just like sending mail to anybody else, but the post office gives it to the military who then takes it from the US to the overseas base or ship
i don't know why they would decline to sell to soldiers, i think the rates don't reflect the leg that the military pays for , ie the overseas part, so i dunno
maybe they think it is hard to track down deadbeat soldiers? it isn't if you contact their command, getting in trouble for debt/not paying bills is bad juju in the military