if you do it the "washington" way, and you pay by credit card, make sure the payment will be authorised. i used a scotiabank card, which was supposed to be okay (according to their security department), but the payment was refused. washington didn't phone the one time when this happened, as they promise they do. i paid a fortune in calls trying to find where the passport was. everything was eventually sent back to me. i had to find a different credit card (banreservas) and send it back again. luckily i did it all a few months in advance and daughter was able to travel on time.
on the other hand, i know someone who couldn't handle the "washington" way and decided to risk posting it direct to the uk. it worked...........my nerves couldn't handle that.