Vendors who supply goods or services in response to purchase orders shall deliver such goods or services in compliance with the terms thereof. When they have performed the obligations which rest upon them under such purchase orders, they shall render invoices therefor, and on such invoices they shall enter the number and date of the pertinent purchase order. Each invoice shall be delivered first to the agency which received the goods or services, which shall prepare and sign a certificate of the following tenor: "The goods and/or services covered by this invoice have been received by this agency and were of the quantity and quality specified in the purchase order and billed on this invoice. Signed " The certificate shall also certify that the computations on the invoice have been audited and found correct.