ORM-D can be shipped by air, but you have to find an outfit that will do it and then fill out a shippers declaration form and pay the fee.
Fedx will accept hazardous good for air transport....but only from approved accounts, meaning that they have demonstrated that they know the hazmat rules and know how to fill out the paperwork and such.
I deal with hazmats by air all the time. It can be done. Not easy for a guy off the street, but it can be done.
Butane could be classified under either ORM-D Air (not many manufacturers do this for butane anymore) or it would fall under UN2037 Receptacles, small, containing gas. You can ship 1kg of UN2037 on a passenger aircraft or 15kg on a cargo aircraft per IATA regulations, although a number of airlines have special rules that limit or forbid this on their aircraft.