Nonsense. Individual spores may be too small to be seen with the naked eye, but it took BILLIONS of spores to contaminate the AMI building. There MUST have been close to a teaspoonful of powder to do that. That is consistent with delivery via a letter and shows that your idea of distribution from "entirely invisible" particles on money is laughably absurd.
No, but it might be consistent with delivery via an envelope. The difference is not subtle. A "letter" goes through postal routes. An "envelope" might simply contain rent money inside it.
Anyway, I'm not claiming that the 9/11 terrorists put contaminated rent money inside an evenlope to give to their AMI landlord. What I am pointing out is that it is plausible for a tenant to put place rent money in an envelope, so if the required amount of anthrax contamination is so large that it demands delivery via envelope, you still can't rule out the rent money theory (well, *you* can rule it out, but that's only because you are wedded to an obsolete, different theory).