Note: 1. You can bill the open SO when you go via Transactions > Sales > Invoice Sales Orders > Check the "Invoice" box next to the Sales order and then click Submit. The system processes the invoice in the background and the record is created when the job is completed.
2. If you bill the sales order by viewing the SO and then clicking on the BILL (or Next Bill/Bill Remaining) buttons, you will be taken to a new invoice, but an error message pops up saying "Warning: this copied transaction uses the following inactive items. You should remove these items before saving if you do not want their balances to change.