code: protected function sendRequest(RateRequest $rateRequest)
      {
          $request = $this->createRequest($rateRequest);
  
          $this->response = $this->getRequest()->request($this->createAccess(), $request, $this->compileEndpointUrl(self::ENDPOINT));
          $response = $this->response->getResponse();
  
          if (null === $response) {
              throw new Exception('Failure (0): Unknown error', 0);
          }
  
          if ($response->Response->ResponseStatusCode == 0) {
              throw new Exception(
                  "Failure ({$response->Response->Error->ErrorSeverity}): {$response->Response->Error->ErrorDescription}",
                  (int)$response->Response->Error->ErrorCode
              );
          } else {
              return $this->formatResponse($response);
          }
      }
docs: Creates and sends a request for the given shipment. This handles checking for
  errors in the response back from UPS.
  
  @param RateRequest $rateRequest
  
  @throws Exception
  
  @return RateResponse