This invention relates to a system for certifying the accuracy of postage payments based on the weights of mail pieces. The expected weight of each mail piece is determined and a mail processing unit receives data relative to the contents to be inserted into an envelope that together form the mail pieces. A high speed weighing scale is located downstream from the mail processing unit to receive the mail pieces after the mail processing unit has inserted selected materials into envelopes. A comparison is made between the expected weight of each mail piece and its measured weight. If the measured weight of a mail piece is found different than the actual weight beyond certain tolerances, it could be rejected. If a large number of mail pieces exhibit a divergence between the estimated weight and the actual weight, the mail processing unit will be disabled and an examination made to determine what caused the discrepancies in the weights. If there is correlation between the anticipated weights and the measured weights, the mail pieces are placed in trays or other suitable containers to be delivered to the Post Office. A history of mail runs and their accuracy with regard to weight is maintained for record purposes.