A computer system utilizes an algorithm that: (a) prepares a table that includes counts of zip code occurrences in a mailing list; (b) prepares a container list that allocates counts to different containers; (c) uses the container list to allocate individual addresses to different mailing containers; and (d) addresses the mailing pieces (directly or by labels) according to the addresses associated with the various containers. Such configurations can advantageously decrease the time required to sort an extremely large mailing into mailing containers according to postal presort levels, among other things by performing the above steps predominantly in the internal memory of the computer system.