I think Tiffie is referring to the 'infinite Delivery Box' trick- where you can have a whole lot of items in your DB.
Basically, send yourself 8 items, go check you DB, so they arrive. You can then send youself 8 more items. Now's the trick- you go back to your DB, still showing the original 8 items- instead of taking them out, you simply hit 'Return'. This sends them back to you. After returning all 8 items, if you pull up your DB menu again, you'll see the 2nd batch of 8 items you sent in your DB. Then you go back to the NPC and send 8 more items (sent box cleared since the 2nd batch of 8 hit your DB).
You can just keep repeating this- not sure if its infinitely or if they cap it. I currently have about 30 items in delivery box limbo. Great way to get some extra storage- just can get a little annoying having to 'return' several DBs worth of stuff to yourself to get to something that was just recently sent.