Ah; I see. I called Ebay at the time and they had no idea. This is the first explanation I've gotten.
These services provide a way for US sellers to send to buyers in the military while delivering their goods to a US address. This ensures that the sellers are protected by delivery insurance & have signature confirmation with carriers such as USPS, UPS, FedEx etc. Apple actually sends to a forwarder and recommends the one I use, APO Box.
http://store.apple.com/us/open/shipping_apo
The forwarder then relabels the package with the service member's APO/FPO address & sends it on its way. This can get rather expensive for service members because they have to pay for shipping, handling, and insurance twice. Although I use Amazon Prime for many purchases, my most recent purchase of a Thunderbolt-to-Ethernet adapter was sold by Apple & therefore had to be sent via APO Box. The shipping to APO Box was free, but the handling, shipping & insurance from there was an additional $16. Obviously it can get rather expensive but there aren't many alternatives as you can't just walk to an Apple Store here and get one.