Snow load for the header will be very significant. I would make sure what is proposed is actually designed by a qualified engineer. For me I would want to make sure it can survive the weight of the one and fifty year wet snow load. If you have enough depth in the header area it should be possible. Most guys that sell or make buildings have no engineers on staff.
I have been pricing large cold storage shed with exactly that. Some companies will do it and some will not. The ones that will are engineered steel buildings and doesn't sound like the cost would be to bad considering