It may depend on what standard of helmet as to what's expected and buying a second hand one isn't something I'd really entertain anyway but I'd be livid if I got one delivered just wrapped up. Box with polystyrene inserts to stop it rattling would be what I'd expect.
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.
It may depend on what standard of helmet as to what's expected and buying a second hand one isn't something I'd really entertain anyway but I'd be livid if I got one delivered just wrapped up. Box with polystyrene inserts to stop it rattling would be what I'd expect.