Wardpike's gluing method described above is the way to go. You should be able to do it yourself with little effort. Make sure you apply the glue to all broken surfaces and line everything up as perfectly as possible - then clamp and wipe the excess glue. Go to
this site and scroll down to the section on "Broken Headstocks". Read through those procedures, and you should have enough knowledge to successfully repair your SG.
The break on your guitar is classic and happens to many Gibsons for the reasons other posters have stated. I have repaired at least five breaks like that, and they all have held up very well over the years.
Next time you ship a guitar, you need to make sure it's in a crush-proof case - you also need to bubble-wrap the neck and suspend it between the top and bottom of the case. Most cases are for protection when you're carrying them or transporting them by hand. If they are to be shipped, they need to really be protected.
Good luck!
Dean