Section 8. An itemized statement of the actual and proper cost to each commissioner for transportation and other necessary expenses incurred in the performance of his official duties shall, on the first day of each month, be approved by a majority of the county commissioners and certified by their clerk to the county treasurer who shall reimburse them. The director of accounts shall audit such expenses in the course of his examination of the books and accounts of the county treasurer pursuant to section forty-four of chapter thirty-five.