OK, guys, I'm working for the Xmas season as a temp worker for the US Postal Service. For the past few days I've been helping sort the 2nd & 3rd class mail (read: opening mail sacks) and have learned a few things that will help you all pack your trains for safe shipment in the case of buying or selling online.
The maximum size we can machine sort at our location is 10" X 12" X 15". Anything larger than that, or mailing tubes, gets hand sorted.
Machine sorted mail goes up on a conveyor belt about 5 feet off the floor. The sorting equipment is a conveyor about 100 feet long made up of sections with small belts that will kick the packages off to one side or the other to a mail chute. At the bottom of each chute is a box (3' X 4' X 3') for sorting to local post offices. If you happen to have your parcel drop into a new, empty box, it's about a 54" drop to the bottom. Then stuff lands on top of it.
Pack your stuff to survive a 5 foot drop and have stuff land on top of it and you'll be fine.
Jay CNS&M North Shore Line - "First and fastest"