lib/Api/Subscription.php
public function buyOneTimeAddonForASubscription(string $subscriptionId, array $data)
    {
        $response = $this->sendRequest('POST', sprintf('subscriptions/%s/buyonetimeaddon', $subscriptionId), [], json_encode($data));

        return $this->processResponse($response);
    }
public function buyOneTimeAddonForASubscription(string $subscriptionId, array $data)
    {
        $response = $this->sendRequest('POST', sprintf('subscriptions/%s/buyonetimeaddon', $subscriptionId), [], json_encode($data));

        return $this->processResponse($response);
    }
@param string $subscriptionId The subscription's id
@param array  $data

@throws \Exception

@return string