code: def enter_offline_payment(self, transaction):
          
          url = urljoin(self._url, '/transactions')
          transaction.post(url)
          return transaction
docs: Records an offline (external) payment on the invoice.
          Pass in a Transaction object to set the details of the created
          transaction. The attributes available to set are
          (payment_method, collected_at, amount_in_cents, description)
  
          Returns:
              Transaction: The created transaction