a purchase order is basically a way for a business to keep track of purchases it makes. The bookkeeper uses it to track expenses and expected invoices (accounts receivable). It would give your business the appearance of a professional organization, and psychologically may make your customer feel more obligated to the agreement instead of just a quote over the phone. Once the deal is made, just tell him that you will be issuing a company check with a purchase order # of 4721 (or whatever number suits your fancy). By the way, it may not be a bad idea to get in the habit of tracking your expenses, so you may want to incorporate PO's into every transaction you do if you intend to make this a serious business venture.