Wrap it so that it can be dropped from a significant height and not damaged. Seriously. I've had 3 bad experiences with mailed machines. And the common factor in all of them was terrible packaging. Bubble wrap the heck out of it. Make sure all attachments and foot control are similarly wrapped and not able to fly about the box. Make sure the machine is packaged tightly in the box and is stationary in the box. If you hear it move about, it's not safe.
It may be worth the cost to you, to have it professionally wrapped and shipped by a shipping store. It would be insured and if there's any damage you won't be out anything.