I recently spoke with the owner of my LGS and he gets ammo from the secondary market delivered almost every day. Unfortunately, there has also been a rather sharp increase in the amount of ammo that has been "misdelivered" or otherwise gone astray while in transit.
What he recommends is that if you need to ship a small or moderate amount of ammo, do not ship in in the factory boxes. Instead, use a food vacuum sealer and seal up a hundred or so rounds into each bag. That way they won't rattle when handled.
The owner of the shop claims he has not had any ammo go missing that were vacuum packed in this manner because as the boxes are being loaded or handled you cannot tell there is ammo inside by the sound or feel.
Hey, whatever works I guess...