I was looking to purchase a Shoei GT Air helmet. According to the tables, my head size showed that I needed a Med. I had tried a few on at a local dealer and found that the XL is what felt good.
I ended up going to Cycle Gear. Prices are low, same as RevZilla. I would have to pay sales tax, but I was willing because I wanted to try on in person, rather than dealing with shipping.
I discussed the size issue with the manager and he was confident that I needed a large. He ordered one in for me. When I tried it on, I found the cheek pieces were exerting too much pressure. The manager said that he would order me a set of smaller cheek pieces which should solve the problem. These cost about $50. Not only did he not charge me for them, but he shipped them to me free because of my long drive - 60 miles.
After using the helmet with the smaller cheek pieces for several rides, I still found it too tight. I called Cycle Gear and they exchanged the helmet for an XL.
I am very impressed with their service and it is now my goto store.