Hi all,
maybe I am not getting it right, but the whole issue seems much more simple to me. According to DMG, the dust of sneezing and choking appears as dust of appearance. As such, the players will intend to use it as if it was dust of appearence, hence most probably throwing it all around them (to see invisible creatures all around and not in only one direction). This means that they will be stunned as well as, most probably, the opponent(s), given the 20 foot radius (which is more than the 10 ft radius of dust of appearence, on which the players made their assumption/calculations when throwing the dust in the air).
Of course, I am assuming that you don't give a wagon of it. Usually they should have 1 dose of it.
I rather doubt they can willingly purchase it, given that it looks something else!