register in nvms 15 16 is sufficient to permit the franking operation in respect of the required postage charge for a mail item to be performed if the value of credit in the credit register is sufficient the franking operation is continued and the accounting data in the registers is updated to account for the postage charge and the franking indicia is printed however if the value of credit in the credit register is not sufficient to permit the franking operation in respect of the required postage charge to be performed the operation is terminated and the franking indicia is not printed where a value of credit available for use in franking is stored in a descending register the check as to sufficiency of the credit available is effected by a determination of whether the postage charge is less than the credit value where a total value of credit is stored in an ascending credit register the check as to sufficiency of credit is effected by a determination of whether the total value of credit is at least equal to the sum of the postage amount and the accumulated total value in the tote register as described hereinbefore the print head module 22 includes an ink supply accordingly when the ink supply is exhausted the module 22 must be removed and replaced by a new module containing a full ink supply removal of the used module and replacement by a new module needs to be effected by a user of the postage whenever the ink supply becomes exhausted as a result if no protection is provided access.