Although a typical purchase order may not be worded as a contract (in fact most contain little more than a list of the goods or services the buyer desires to purchase, along with price, payment terms, and shipping instructions), the purchase order is a specially regarded instrument regulated by the
Uniform Commercial Code or other similar law which establishes a purchase order as a contract by its nature. Yet despite the nature of the purchase order as a contract, it is common to accompany the acceptance of a purchase order with a legal document such as the terms and conditions of sale, which establish specific or additional legal conditions of the contract. The US
Federal Acquisition Regulation states that purchase orders should generally be issued on a fixed-price basis, but provision is also made for unpriced purchase orders to be issued where "it is impractical to obtain pricing in advance of issuance of the purchase order". In the UK, the
Office of Government Commerce noted with concern in 2010 that "contracting authorities [were] not always raising purchase orders", and that where they were used, invoices were not always being reconciled to purchase orders before payment. Some organisations operate a "No PO, no pay" policy, which means that invoices which do not refer to a purchase order number will be returned to the supplier unpaid. The
City of London Corporation, for example, operates such a policy. == Formats ==