The only way this could possibly work would be if you were offered a huge discount on a helmet, from a wide range, which most bike shops don't stock anyway. Much like saddles, helmet fit is a very personal matter, and even if you wanted a helmet, it would be extremely unlikely that any bike shop could or would stock a range to fit all. Result - lots of cyclists with badly fitting helmets, and/or lots of helmets binned. It is just a non starter. If you want a helmet, buy one that fits, shop around and buy your own. (I do wear one - a Spiuk, because it fits, and it took me a long time to find that - how many bike shops stock them?)