A method and system are provided for uniquely assigning identifications for a plurality of mail pieces, before physically sorting the mail pieces that will subsequently be mailed. Each of these identifications includes a sequential part, and each of the mail pieces includes postal destination information indicative of a destination area sort level. Preferably, not only that destination area sort level is included, but also at least one wider destination area sort level. The present invention entails determining whether a current mail piece has postal destination information indicative of the same destination area as a previously processed mail piece. If the destination area is the same, then the sequential part is assigned to the current mail piece, such that the sequential part is monotonically different from the sequential part of the previous mail pieces having the same destination area.