Used USPS, (I think the only option) and has worked well 10+ times!
Ship to my sister-in-law quite often (she's in NE Italy) via USPS. Seems that we are charged domestic rates and/or some type of discount is applied.
Don't send chocolate Easter Bunnies @ Easter (they arrive as Easter Blobs) and since the anthrax scare items in powder form (we used to send various manicure supplies) are not appreciated by the receiving staff @ the base.
IT SUCKS! The APO system stinks for larger audio gear. Priority mail only gets you as far as the APO terminal in the US, then you are on your own. Same with insurance. I had to file a claim and it takes months. You had to wait 90 days to file and then wait 60 or so more to get anywhere. It was perhaps the worst deal I ever did.
I feel badly for the soldiers who are stuck with the system but the last thing a high end piece needs is a 3 week journey on a ship, then dropped off a helicopter and plugged into garbage voltage overseas.
DON'T DO IT!
Hate to take exception to Elevick, but...I was overseas in the UK for three years and had a number of audio items shipped to me via USPS to my APO address, with no problems. I normally recieved packages within a week to 10 days, although Kosovo will no doubt take longer.
You might insist on advance payment and insist that the buyer handle any claims if there is a problem, but I would not be deterred from shipping to an APO address. You will be limited to 70 pounds and some dimension limits on the package.