the line 

if( !is_null($this->_item_tota) )

probably should be

if( !is_null($this->_item_total) )


in function

  public function get_item_total()
  {
    if( !is_null($this->_item_tota) )
      {
	// item total has been overridden.
	return $this->_item_total;
      }

return ($this->get_unit_price() + $this->get_unit_discount()) *
$this->get_quantity();
  }