Summary
You may set some Work Orders to $0.00 for the work, but when it comes time for invoicing you want the price available. You can filter invoices that were posted at $0.00
Key Point
To search for Work Orders that never had a price and were made into invoices:
From the ribbon bar, click Sales
Click Invoices
Click the Total column header one time to show the $0.00 invoices at the top of the list.