To elaborate, a PO is an offer from the buyer to the supplier. If accepted by the supplier (within a reasonable timeframe) the PO becomes a contract. Logically, this PO will have zero value if it's not accepted by the supplier.
Write the contract in six steps Start with a contract template. Open with the basic information. Describe in detail what you have agreed to. Include a description of how the contract will be ended. Write into the contract which laws apply and how disputes will be resolved. Include space for signatures.