A state agency that purchases road construction materials may pay for road construction materials delivered in a vehicle that exceeds the maximum gross weight authorized by law for the vehicle an amount computed using the lesser of:
(1) the actual weight of the load; or
(2) the weight determined by subtracting the weight of the vehicle from the sum of the maximum gross weight authorized by law for the vehicle and the tolerance allowance set for the gross weight of that vehicle by Section 621.403, Transportation Code.