How well are new helmets packaged?
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.