If you do it through ebay, then a customs form will be populated with your postage and a little more information (and signature) from you. So pretty easy. If you are worried, then go to the P.O. and they will walk you through it.
That being said, orders to APOs will probably cause dings to your shipping time. The last two things I sent to APOs took weeks. One is still on the way and the other (I think) arrived but never got its final scan so still shows up as "in transit". That being said, I can't bring myself to add APO's to my "will not ship to list."