Okla. Stat. tit. 11 § 17-114
To facilitate the payment of vendor invoices and contract estimates the municipal finance officer may design a uniform jacket to be used by all departments and divisions of the municipality whereon shall be provided summarized information relative to the enclosed invoices or contract estimates, together with a space for the approval of the head of the department or division approving said vendor invoices or contract estimates for payment. Vendor invoices and contract estimates may be accepted by the municipality in lieu of the claim form previously required in the same manner as commercial invoices are paid. If utilized, vendor invoices and contract estimates shall be filed with the department or division receiving the merchandise or services in the same manner as invoices are filed with commercial firms. Upon receipt of invoices or contract estimates the head of the department or division or his authorized agent, may approve said documents for payment by executing a certificate of delivery or acceptance of the goods or services. Whereupon, the authorized official of said agency may approve said invoices or contract estimates for payment by enclosing the invoice or contract estimate in a jacket provided for such purpose and affixing his or her approval in the space provided on the jacket.
Okla. Stat. tit. 11, § 17-114