Abstract: A method and apparatus is disclosed for combining multiple small stacks of flats mailpieces into a single large stack of mailpieces and then transferring the large stack to a standard flats mail tray, all while maintaining the sequence order of the mail in the accumulated stack. The apparatus is comprised of three primary subsystems: a bridge conveyor, a stack accumulator, and an output tray station. The bridge conveyor carries mailpieces from the exit conveyor of a mail processing machine such as a collator, to the stack accumulator. The stack accumulator combines small stacks of mailpieces into large stacks in a desired sequence. The output tray station provides support for an empty tray as the accumulated mail stack is transferred to the tray, and then releases the filled tray in a controlled manner.