shipping a transmission is pretty easy.
I use the double-box method. Pack the transmission in a box that's just big enough for the tranny + some packing material (pieces of cardboard, styrofoam chunks, etc) Tape it securely, then put it in another, larger box using plenty of packing/cushioning material in between the inner + outer box. Pay extra attention to things that could possibly poke thru the inner box- like the output flange and the input shaft- wrap them up before boxing.
This works well for me. One thing to keep in mind is that boxes have different weight ratings- look on the bottom of the boxes you plan to use and make sure they are plenty strong enough to handle the weight of the tranny. There is a stamp on most boxes that shows the max weight capacity of the box.
You can also use one of those Rubbermaid type containers that's the right size. Available for less than $10 and they are pretty tough. Use plenty of packing material inside to cushion the tranny, and drill some small holes around the top edge so you can secure the top to the box with zip ties. I've also heard of people who wrap the tranny in a plastic bag, then spray the bottom of the rubbermaid container with some of that expanding foam insulation (Great Stuff or similar), drop the bagged tranny in, and then "foam" it in place. Sounds like a good way to do it, and not too expensive.
Bret