A proper invoice submitted by the contractor will be required to initiate any payment by the corporation, except where the contract provides that the contractor will be paid at pre-determined intervals without having to submit an invoice for each such scheduled payment. For the purpose of determining eligibility for payment of interest and subject to the provisions of sections 801.5 and 801.6 of this Part, the date by which contract payment must be made in order for the corporation not to be liable for interest payments will be the payment due date specified in the contract.