code: def set_vads_payment_config(self):
          
          self.vads_payment_config = tools.get_vads_payment_config(
              self.payment_config, self.custom_payment_config.all())
docs: vads_payment_config can be set only after object saving.
  
          A custom payment config can be set once PaymentRequest saved
          (adding elements to the m2m relationship). As a consequence
          we set vads_payment_config just before sending data elements
          to payzen.