public function confirmationUrl()
    {
        // Begin the charge request
        $charge = $this->api->rest(
            'POST',
            "/admin/{$this->plan->typeAsString(true)}.json",
            ["{$this->plan->typeAsString()}" => $this->chargeParams()]
        )->body->{$this->plan->typeAsString()};

        return $charge->confirmation_url;
    }