<standing on a high building and thowing it out a window over a crowded area, riding a bicycle through a crowd with an open container blowing in the wind, etc, etc. Powder in a letter seems quite ineffective if your goal is to infect the maximum number of people.

Based on what I have learned, it would require like a 50 gal. drum of the stuff to be effective in the way you describe.

It is hard to know what they had in mind. I imagine they would have liked more than one fatality but if they expected a large number of casualties it makes whoever they are look pretty stupid. I like to think of them as stupid but stupid or not they have accomplished a lot with their wimpy effort.