One question I have is about the actual mailing.
Consider: You've finished up in your BL4 facility, and now it's time to drive up to Princeton and mail the letters. What do you use to contain the letters during the trip? Now it's time to park and walk to the mailbox and drop the letters. How do you physically do it? A space suit would be highly indiscreet and might scare the sorority girls if any are around. Maybe you just take plenty of Cipro. Would that be sufficient?
Maybe you get an unsuspecting accomplice to do it for you.
After all
if you are out to kill people
what difference if one more bites the dust?