I'm eager to find out about this too!
I've got a dozen boxes set for 7 or 8 contacts I put together in the last four days and I wanted to get them all out on Friday... Looking online it SEEMS as though I can fill out customs forms online (they even make them APO/FPO-specific, I was looking at it going "For goodness sake, how am I gonna select a city and state?" but the drop-down menus for City only list "APO and FPO" and for State it lists "AA, AE, and AP" so they actually got that covered!) And it's slightly cheaper, it seems - $11.50 versus $11.95 per package... AND my mail carrier will pick them up at my house on his regular route so I don't have to haul them all to the Post Office.
Hmmmmm...
If no old hands have any warning advice about this (and if so, please tell me!), I'm REALLY going to have to get up the nerve to do this - saves me time, money, gas, and energy!