includes/class-freemius.php
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,
            ) );
        }
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,
            ) );
        }
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 , ) ) ; }
Add-on checkout URL.

@author   Vova Feldman (@svovaf)
@since    1.1.7

@param number $addon_id
@param number $pricing_id
@param string $billing_cycle
@param bool   $is_trial

@return string
Add - on checkout URL .