A justification insert block is adapted to be inserted into a flat mail sorting or storage container, bin, or tub and comprises an upper planar surface which is downwardly inclined from a first corner region toward a second diagonally opposite corner region. In this manner, when the justification insert block is inserted within the flat mail sorting or storage container, bin, or tub, two edge portions of mail pieces are automatically justified against two upstanding side walls of the sorting or storage container, bin, or tub so as to maintain the mail articles or pieces in a predetermined order within a stack of flat mail articles or pieces.