Members of the Commission who are Members of Congress, officers of the executive branch of the Federal Government, Governors, or full-time salaries officers of city and county governments shall serve without compensation in addition to that received in their regular public employment, but shall be allowed necessary travel expenses (or, in the alternative, a per diem in lieu of subsistence and mileage not to exceed the rates prescribed in subchapter I of chapter 57 of title 5), without regard to subchapter I of chapter 57 of title 5, the Standardized Government Travel Regulations, or section 5731(a) of title 5, and other necessary expenses incurred by them in the performance of duties vested in the Commission.