I think what I will do is if it is going to be more than the 5.50 flat rate box than I will just use that. So It wont be more than 5.50 and I would let you know before I send.
You can also take the actual hard drive out of the shell and put it in a slim hard drive case (which you can get on Amazon for around $5) and then you can just slide that into your Xbox 360 Slim ; )