SECTION 13. Increase of services or supplies specified in contract

The governmental body may increase the quantity of supplies or services or both specified in a contract provided:

(1) the unit prices remain the same or less;

(2) the procurement officer has specified in writing that an increase is necessary to fulfill the actual needs of the governmental body and is more economical and practical than awarding another contract;

(3) the parties agree to the increase in writing; and

(4) the increase in the total contract price does not exceed 25 per cent but a contract for the purchase of gasoline, special fuel, fuel oil, road salt or other ice and snow control supplies shall not be subject to this limit; and

(5) the governmental body, with the agreement of the contractor, may reduce the unit price for supplies or services or both specified in a contract to be paid by the governmental body at any time during the term of the contract or when an option to renew, extend or purchase is exercised.