code: public function buyOneTimeAddonForASubscription(string $subscriptionId, array $data)
      {
          $response = $this->sendRequest('POST', sprintf('subscriptions/%s/buyonetimeaddon', $subscriptionId), [], json_encode($data));
  
          return $this->processResponse($response);
      }
docs: @param string $subscriptionId The subscription's id
  @param array  $data
  
  @throws \Exception
  
  @return string