If I remember correctly, you have to fill out an initial application online and pay a fee. Then you can either make an appt at tha nearest consulate/embassy that processes visa or have an approved courier bring in the docs for you to the nearest consulate. I don't remember snail mail being an option, but then I was under a lot of pressure to get it done in a weeks time, so I didn't really look into it.
I do remember the couriers having their own queue at the NYC consulate and they seemed to know people there. Seemed reliable enough, but like I siad I was pressed for time and it was safer/easier for my company to fly me to NYC to do it in person.