You don't need to Invoice from the Estimate, to use an Estimate as a comparison reporting tool for PO completion. If you need to charge the customer from actual Billable time, go ahead. The point of Estimate is to lay the baseline for that project or budget. Do not Progress Invoice from it, if you mark time as billable and use that with the Add Time & Costs Function. You still have what you need for Est vs Actuals reporting. And the Timesheet reporting exists, such as Time by Job by Name by  Item, or By Job by Item by Name. Add the Columns for Billable, Not Billable, Billed, and Estimated. This lets you compare quantity across that row and these columns.