I think I got it.

The item list will be set up by sq footage, so a 24x18in item (3 sq ft) would be a different item than a 36x24in item (6 sq ft) even though they may be otherwise identical. They would enter the 3 (or 6) in both the cost field and the Adjustment value field when they create the item (one-time setup), and then they won't have to think about it every time they invoice.

Whew, I am showing them Invoices tomorrow.

Thanks for helping me think it through. I just hope they don't have too many different print sizes.

Steve