I seem to remember about ten years ago that the USPS wanted to go to standardized envelopes to facilitate automatic sorting. Maybe that idea will be revived, only with some "enhancements" that might confound this kind of biological attack. (Special seal? Biocidal agents?)
Probably the only ones that have done a dose-response curve for Anthrax spores is the U.S. Army, and they're not talking.
I couldn't find much with a google.com search. Maybe I should try again with that additional information you provided.