My understanding (and likely to be highly incorrect) is that if flight duty pay is paid at a reduced tax rate it is because it is used for living allowences (e.g. food + drink) that would normally be available to the crew member at home but is not due to the nature of the jobs.
I'm not sure if commission is covered under these rules or not.
Either way I wish them luck as if it happens to one, it will filter down for all.