I just shipped a transmission to AA Transaxle yesterday, on behalf of
Matthew Pollard.
Matthew recommended: 4" of bubblewrap, and two layers of cardboard. I
decided to do that, and it worked out pretty nicely.
I used the bubblewrap that has larger bubbles, and my cardboard was
once a couple of bike boxes. I chopped them up to fit the heavy lump of
bubblewrap snugly, and used 3 rolls of 3M Super tape. The whole thing
held together VERY well- we'll see if there are any complaints on the
receiving end- and only weighed in at 99 pounds, which was pretty cheap
to ship via FedEx Ground.

-Simon