I'm the AC you're replying to..
It kinda works but... Unless the bomb itself was kinda small, it'd be in a package (even a courier one) more than a letter. Letter bombs are old, but from what I have seen of them they were still at least the size of a pack of cigarettes. Well, thinner, but larger length x width. And the article does say it was in a "padded envelope". (was also sent via international mail supposedly, missed that first time I read the article!)
BTW, this was an accounting firm who received the item. They get all sorts of stuff through the mail at times, probably packed in all sorts of ways as well. A padded envelope wouldn't exactly be weird though having the probably detectable shape might've been. Also, an accounting firm would know the importance of either returning or forwarding mis-addressed stuff.
Lots of people still send stuff through the post, although much much less than used to. Some of that stuff is urgent and some is valuable. I'd say "ignore it at your peril" but obviously in this case that wasn't so.
And still the question of why it took them so long to get caught up on their packages....
It may be all in innocence, but it does smell a bit like that meat I forgot to put away from the BBQ last week that spent 3 days in the back yard in the glass covered dish..