One can easily increase the discount from zero and, for each discount level, calculate several  (1) the total revenue lost by the postal service, which is simply equal to the discount level multiplied by the presort volume; (2) the total cost incurred by mailers or by mailing organizations, which is the area under the supply curve; and (3) the total savings of the postal service because it does not have to do the sorting and processing, which is the area under the cost curve.
They are able to increase the discount from zero with ease.