The USPS has charts and graphs and detailed info on it.
Based on how you sort it, how you package it, how you load it, wheter or not it is an open piece needing tabs to hold it close of a spot of glue to hold it closed... total weight of the entire mailing and who picks it up from your facility or if it is dropped off at a pre-sort facility.
Most Fulfillment/Pre-sorting companies do all that for you.
The Dmoestic Mail Manual tells all:
http://pe.usps.com/text/dmm300/dmm300_landing.htm
Based on how you sort it, how you package it, how you load it, wheter or not it is an open piece needing tabs to hold it close of a spot of glue to hold it closed... total weight of the entire mailing and who picks it up from your facility or if it is dropped off at a pre-sort facility.
Most Fulfillment/Pre-sorting companies do all that for you.
The Dmoestic Mail Manual tells all:
http://pe.usps.com/text/dmm300/dmm300_landing.htm