I don't know about quests but you get another note from a dead courier in Primm and a wrecked mess that is oddly directed towards you.
Reno
I've seen speculation that this might be part of some future DLC, I don't know, know.
I don't recall any mention of courier missions, though, so I'm guessing you're just imagining things, though I guess it could also have been something that the designers had originally meant to add, but decided to scrap the idea for one reason or another.
As far as the numbered delivery order,
Spoiler I recall that the player is told that Six couriers were hired to deliver different items, which explain the "6 of 6" thing.
Though I've only every found 4 of 6, and I assume the others just don't exist in the game. I don't worry about it too much, it seems to me like going into any more detail on the matter isn't really important to the story.