You are not exercising the privilages of your commercial certificate to be a CFI. This is why you can do it with a 3rd class medical. Flight instruction is the only job you can get paid to do and it not be considered commercial flying. If it was you would be in violation of the limits placed on a commercial certificate. You would be "holding out" which can even be described as "general knowledge you are for hire". Call your local FSDO to ask.

Well you can get pait one other way.... that I know of. Who know's what it is? Hint:You can do it with only a private pilot.