Purchase order generator
What a purchase order does
It tells a supplier what you are buying, at what price, delivered where and by when, before the money moves. For a one-person business it is the document that stops a merchant's invoice arriving with a different quantity or a price that was quoted differently on the phone: the order is what was agreed, in writing, with a number on it that the invoice has to quote back. Purchase orders, work orders and job sheets sets out what each is for and when a customer will send you one.
What goes on it
Your details and the supplier's, a number from its own series, the date, a delivery address where that differs from your own, each item with a quantity and a unit price, and any terms that matter: delivery by, account number, who signs for it. The generator carries all of that, and lets the supplier's details come from the same list as your clients, since the builders' merchant is both.
When the invoice does not match
The order is what you check the supplier's invoice against: quantity for quantity, price for price. A merchant's invoice that quotes your order number and differs from it is a query you can raise in one sentence, with the two documents side by side; one with no order behind it is a memory of a phone call. Keep the order with the invoice it became, for as long as you keep the invoice.
Numbering the order
Orders take their own series, PO-0001 onwards, kept apart from invoices so neither run gains a gap from the other. Quote the number on everything that follows: the delivery note, the query, the expense. It is the thread that ties a purchase together from the phone call to the receipt.
Then the expense
When the goods arrive and the bill follows, the order becomes the expense in one press: the supplier, the net, the tax and the job it belongs to, so the job's real cost is a figure and not a memory. What a job actually left you is the reason to bother.