*Add New - adds the invoice item regardless of whether it exists or not.
*Add New and Subtract - add an invoice item and at the same time add the same item with a negative value (to a different invoicee) making no nett change but effectively moving an item to a different invoicee
*Update or Add New - updates the invoice item if it exists, otherwise adds it as new.
*Update only - updates the invoice item if it exists but will not add as new.
Invoice items are matched on the description and optionally status if specified.