31 C.F.R. § 206.3
The billing process is considered an integral part of an effective cash management collection program. In those situations where bills are required and the failure to bill would affect the cash flow, bills will be prepared and transmitted within 5 business days after goods have been shipped or released, services have been rendered, or payment is otherwise due. An agency may prepare and transmit bills later than the 5-day timeframe if it can demonstrate that it is cost-effective to do so. In addition, the bill must include the terms and dates of payments, and late payment provisions, if applicable. Terms and dates of payments will be consistent with industry practices. I TFM 6-8000 describes detailed billing policies, procedures, and industry standards for agencies.
31 C.F.R. §206.3