How well are new helmets packaged? I am considering buying a new helmet online, which means it'll be delivered to my door by courier.  In my experience, couriers can be careless with parcels.  Should I be concerned about this, or are helmets packaged so that they're safe even if knocked about a bit?

In case it matters, the brand of helmet I shall be buying is Specialized.
Helmets are like hats and shoes — everyone has a different shape head or foot and so the standard sizes really don’t apply. Some heads are pointy, some are flat. Different helmet companies use different head forms — and some even use different shapes between their different helmet line. So a ‘L’ for one helmet might not be the same as an ‘L’ in another. 

So I’d recommend going to your local bike shop and get a properly fitting helmet. You’ll be surprised just how much better it fits on your head and how much more secure it is.