function checkout_url(
            $billing_cycle = WP_FS__PERIOD_ANNUALLY,
            $is_trial = false,
            $extra = array()
        ) {
            $this->_logger->entrance();

            $params = array(
                'checkout'      => 'true',
                'billing_cycle' => $billing_cycle,
            );

            if ( $is_trial ) {
                $params['trial'] = 'true';
            }

            /**
             * Params in extra override other params.
             */
            $params = array_merge( $params, $extra );

            return $this->_get_admin_page_url( 'pricing', $params );
        }