Section 28. They shall serve without compensation, but each commissioner shall be paid by the commonwealth the amount of his actual traveling and other necessary expenses incurred in the discharge of his official duty, after an account thereof has been audited by the commissioners. They shall keep a full account of their expenditures. They may make a contribution each year toward the expenses of the conference of commissioners on uniform state laws, to be paid from the appropriation for the expenses of the commissioners at the discretion of the attorney general.