Is the pot decent? If so, I'd ship the pot and tree separately. The pot, in addition to being heavy and fragile, will also be a liability to the tree if the box is turned upside down.
I had two trees sent to me recently from Ed Clark at Round Valley Nursery. The trees were removed from their pots, and the root ball was wrapped completely in shrink wrap. Then a 4" piece of plywood was put on the bottom of the wrapped rootball, and the entire thing was shrink-wrapped again. Then he built a wooden frame that fit perfectly within a thick shipping box - bottom and top were plywood, the supports/legs that attached top and bottom were long 1" square pieces of scrap. He put each tree in the frame, and drilled through the bottom of the frame with decking screws - and into the piece of wood that was shrink-wrapped to the bottom of the root ball. Slide the frame into the box - and done. No other plastic, packing peanuts, nothing. I think they could have dropped those trees out of a plane and they'd be fine - and the foliage was not bruised or limp when the two trees arrived - because they were not encased in plastic or touching anything.