Agree with pretty much everything above. Peanuts and bubble wrap. . . as much as will fit in the box.
If the box is larger than 12x12x12, shipping goes up considerably.
I also wrap the helmet in a layer of quality paper towels before packing just so no styrofoam or plastic
makes contact with it. I've received stuff before that was wrapped in bubble wrap and had faint bubble
impressions on the surface. Heat can do this, or wrapping the item before the paint has fully flashed out.