In order to initiate payment pursuant to a contract, a contractor shall be required to submit a proper invoice to RIOC, except where the contract provides that the contractor will be paid at predetermined intervals without having to submit an invoice for each scheduled payment. A proper invoice shall contain a written request for contract payment and shall set forth the description, price and quantity of goods or services delivered or rendered in such form and supported by such other substantiating documentation as RIOC may reasonably require. For purposes of these regulations, receipt of an invoice means (a) the date on which a proper invoice is actually received by RIOC, or (b) the date on which RIOC receives the purchased goods, property or services covered by the proper invoice, whichever is later.