public function subscribeExistingCustomerToPlan($customerId, $planId, $parameters = [])
    {
        $data = [
            'customer'      => $customerId,
            'plan'          => $planId
        ];

        if ($parameters && is_array($parameters)) {
            $data = array_merge($parameters, $data);
        }

        return Subscription::create($data);
    }