Haha, don't believe Seagate.
I have had 3 defective Seagate 1TB Hard drives and I always just shipped it back with 1 layer of bubble wrap and a simple padded envelope from Canadapost. They don't care how you send it to them. They're well aware of their manufacturer defects on their hard drives and aren't willing to start any problems due to the fact that they can lose some serious reputation.
Trust me, you don't need a box.