Abstract: The flat oversize mail piece includes an envelope of expandable nature, a flexible insert within the pocket of the envelope and an item that may be rigid or non-rigid. In one embodiment, the insert is provided with a cutout that is asymmetrical to one side of the insert and envelope to matingly receive the item being shipped. In another embodiment, the insert is a thin plastic tray having a recess to receive the item being shipped and a plurality of cavities interconnected by thin webs that impart flexibility to the package.