DR Jennifer maybe the store can exchange your helmet for the correct size at the same price. Next time ask them to remove the cardboard while you try it on. It is terribly annoying all the attached crap we have to deal with when trying on clothes. I've insisted on items being removed for me that were simply too bulky to work around.
I looked for someone in the department for quite a while. There was nobody. This was the only size for that price. I scanned 4 helmets and they were all different prices (and not marked).
I wanted to get them to remove the cardboard. You could still try it on. I just couldn't feel that it was tight with the cardboard there.

It's no biggie. I will return it (it was Walmart). I just thought it fit.
I actually need to go to a different Walmart tomorrow. So I will bring it back there. ANd maybe there will be other helmets at that one. Also I was looking for a certain kind of sandal. So maybe this Walmart will have them.