Each commissioner shall receive $210 for each day actually served and expenses incurred in the performance of his or her duties paid at the same per diem rate as members of the Legislature: Provided, That the presiding commissioner shall receive an additional $50 for each day actually served. In addition to the expense per diem, each commissioner may, when using his or her own vehicle, be reimbursed for mileage. The number of days served by each commissioner shall not exceed one hundred twenty in any fiscal year, except by authority of the President of the Senate and the Speaker of the House of Delegates: Provided, That in computing the number of days served, days utilized solely for the exercise of duties assigned to  commissioners by this article and the provisions of article two-a of this chapter shall be disregarded. For the purpose of this section, time served shall include time spent in the hearing of claims, in the consideration of the record, in the preparation of opinions and in necessary travel.