I know that they confirmed spores at the Boca post office so we can conclude that the AMI letter was probably mailed from Florida. The evidence for the other letters strongly indicates that they were mailed from New Jersey probably by by cells operating from Trenton or Jersey City.
The letters were "postmarked" in New Jersey and spores were found in and around the machinery involved. No spores were found anywhere else in New Jersey. The only place we have any mail acceptance activity showing contamination is Florida.
Single piece rate First Class Mail could be mailed anywhere and still be "postmarked" in a different place. This is not unusual!