docs: def csv_line_items(self):
          '''
          Invoices from lists omit csv-line-items
  
          '''
          if not hasattr(self, '_csv_line_items'):
              url = '{}/{}'.format(self.base_url, self.id)
              self._csv_line_items = self.harvest._get_element_values(url, self.element_name).next().get('csv-line-items', '')
          return self._csv_line_items