The early report on the envelopes (within days of the attacks) was that the flaps were not sealed, rather they were tucked in "loose". This is why the powder could be leaking on equipment during handling and contaminating adjacent envelopes (not always to a lethal level, but to a measurable degree).
The spores were able to infiltrate the paper and go right through the walls of the envelopes anyway.
My scenario would have Mr. Atta's friends dropping off 3 or 4 envelopes in local curbside collection boxes down there in the Boca Raton area.
Normal postal operations would then serve to bring those letters together in the same stack of "empty" trays. Here's how:
1. All the collection boxes were in a wealthy neighborhood - theirs! The USPS frequently places collection boxes in such neighborhoods even though there is rarely any mail dropped in them.
2. Each collection box has a large "flat tray" in the bottom. This is that larger postal tray made out of corrugated polypropylene that the general public likes so much - for gardening, in the garage, trash collection, etc.
3. When mail is dropped in the top of the collection box it drops into the tray in the bottom.
4. The carrier or collection route employee replaces the tray, taking the supposedly "full" tray and putting it in the back of the collection vehicle. That way no letter is missed - and you better believe, those rich old people with nothing better to do than watch the collection box know if the carrier has done the job correctly - and if not they will instantly call postal management and complain.
5. The carrier stacks the trays taken from the collection boxes up in her vehicle. At the end of the route, those trays are transferred to the collection platform.
Since these trays are virtually empty they will be mistaken for empty mail transportation equipment (MTE). Because Florida is a mail destination area and generates little of its own, lots and lots of postal equipment would pile up down there unless they shipped it back North. Those empty trays were stacked with other trays on a pallet, then wrapped in plastic sheet and shipped North to places that needed more empty trays - e.g. Philadelphia BMC which, in turn, supplies equipment to Trenton.
As the stack of trays was "worked", the letters were found and tossed into 010. That particular stack of trays was "worked" down over a period of days which is why I believe the trays were sent to a major mailer in the Trenton area and not directly to a postal facility. Later unexplainable contamination occured when those very same contaminated trays were sent around to other jobs.