OK, i've shipped just one window, and was a nervous wreck until it got there, but heres what I did..
I found a box that had ample room inside for the glass, and put the window in a large bag to protect the glass. Then I placed styrofoam blocks in the bottom of the box and placed the window inside. Then I took it to a jobsite where they were doing sprayfoam insulation on a new house and had them "fill" the box by spraying the foam inside the box. It expands like all get out, but set up quick and didn't pucker the box at all.
I knew the insulators and was able to get that part done for nothing, but I can almost guarentee that if you went to an insulation company that did sprayfoam, they would give you a nominal price for filling the box. Even better if you are in the trades, or know where they are insulating a house, I'd bet they would fill the box for beer money. I flipped them a $10 spot and everybody was happy!!!
I am very cautious about shipping items, and would not consider shipping another window unless I could do this procedure again!! JMO, Geof