Resource with "Qty to Ship"
Remaining with the Sales Order also gives you some flexibility. Since the invoicing is only allowed after "shipping", if there are labor hours that are entered but not yet ready to be billed you can blank the Qty. to Ship and those lines will remain open until that phase of the project is ready to be billed.
As a side note, if you are estimating your projects prior to the billing you should consider using the Jobs module. This will allow you to estimate the project, track your labor time, and bill directly from the Job. This gives you greater analytical options for tracking estimate vs. actual and overall project profitability.
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Best Regards,
Ben Baxter
Accent Software Inc