Pay for Payment for WooCommerce by kybernaut.cz

if (class_exists('WOOCS')) {
    global $WOOCS;
    $cost = $WOOCS->woocs_exchange_value(floatval($cost));
}

This edit is inside another plugin's file, so a plugin update will undo it. Write the change down, and re-apply it after every update of that plugin — or ask its authors to include the fix on their side, which is the only version that survives. If the change is inside FOX itself, tell us: a fix that has to be re-applied by hand is a bug on our side, not a solution.