The postmarks indicate different mailing dates: the 1st (as I recall) on 9/18;the 2nd on 10/08 .
As is all too often the case, perfectly good mail got caught up in "Empty Equipment Otherwise Believed to Be Empty" and hauled in a truck from South Florida, where empty equipment piles up, to New Jersey, where empty equipment is needed by major mailing companies. When the mailer's folks found the letters, they dropped them in a tray they keep on hand for such items, and took it to the post office with the rest of their mail in the afternoon.
That's when the letters got canceled. That's also why they didn't go through a collection box in New Jersey, or get placed in a residential delivery box for pickup by a carrier.
We can even explain the New Jersey carrier's infection this way ~ she got it from cross contamination from the letter tray that'd previously held an anthrax letter. That would have happened in the morning when she got that tray filled with mail for delivery on her route.
The FBI spent millions and millions and millions of dollars on fruitless searches because they never bothered once to find out how single-piece rate mail of this kind travels and what can happen to it.