A mailing machine that optimizes throughput by reducing the amount of time necessary for the PSD to generate the digital signature and indicium for each mail piece is provided. The debit operation performed by the PSD, i.e., adjusting the PSD registers, is separated into three different sections, a pre-debit operation, a perform debit operation, and a complete debit operation. In addition, the calculation of the digital signature can optionally be pre-computed, or, alternatively, computed in stages, i.e., partial signature calculation. Utilizing this granularity, the cryptographic operations associated with generating the digital signature can be shifted between the three debit operations such that the execution time of the time critical portion of the debit operation (perform debit) can be optimized to meet the performance requirements of the mailing machine in which the PSD is deployed.