Saving the Invoice or Shipper 

Once you have entered the required information for the shipment being processed, you may save the invoice by pressing the Save Button that is located on the Standard Tool Bar.  At this point, the system creates a new invoice header record and line item record(s) for the shipment, it updates the sales order, inventory items, serialized inventory items, and other information associated with the shipment, and it posts the invoice (if appropriate) to the general ledger.  The system then displays the new invoice number that was created and clears the Order Invoicing screen.

More:

Resetting a Picklist