The following statuses are available on the pick list

STATUS_NOSTOCK = -99          ' No Stock
STATUS_SPLIT = -88 ' Split
STATUS_ALERT = -77 ' Alert to Review
STATUS_RETURN = -66 ' Return of sale order
STATUS_CANCELLED = -55 ' Cancelled sale order
STATUS_RTP = 0 ' Ready to Pick
STATUS_PARTPICKED = 1 ' Partially Picked
STATUS_PICKED = 2 ' Picked
STATUS_PARTPACKED = 3 ' Partialy Packed
STATUS_PACKED = 4 ' Packed
STATUS_SHIPPED = 5 ' Shipped
STATUS_CLOSED = 6 ' Closed, Imported into MYOB
STATUS_FINISHED = 7           ' Closed and Locked

You can use the following to set the status.

CASE WHEN dbo.PKL_PickList.PKL_Status >= 4 and dbo.PKL_PickList.PKL_Status < 5  THEN 'Despatched'  WHEN dbo.PKL_PickList.PKL_Status >= 5 THEN 'Invoiced' ELSE 'OnOrder' END AS InvoiceStatusID