I suggest, Double box or single box with thick bubble wrap. I've posted a few helmets like this without a problem. Imagine if you will that if the helmet is 'floating' inside then it won't be affected if the box is dropped. Like a bone china teacup in a large box surrounded by bubbles.
well £180 helmet, sold for £50. Good used condition. I'll build a box and pack it in.
Cheers!