function addon_checkout_url(
            $addon_id,
            $pricing_id,
            $billing_cycle = WP_FS__PERIOD_ANNUALLY,
            $is_trial = false
        ) {
            return $this->checkout_url( $billing_cycle, $is_trial, array(
                'plugin_id'  => $addon_id,
                'pricing_id' => $pricing_id,
            ) );
        }