Wow, I guess my decision to go with the readily available invoice order module was a good one, then. I ended up changing the text on the "Instructions" textarea instructing customers to use it for comments, instructions, or purchase order numbers. With that field set to appear on every piece of order-related paperwork that I could find, my client seems satisfied with this solution.

Thanks for doing the diff, john_dude. Where the heck did you find the Purchase Order module, anyhow?

Oh, and my wife explained what a Purchase Order amounts to - an invoice with a code the customer supplies, for their own tracking purposes. Otherwise, it's just a basic invoice: products delivered along with a request for payment after the fact.

Frank