sort_fc_products_by_id.php

<?php

add_filter('fluent_cart/shop_query', function($query, $params) {
      // Change default sort to ID-based (instead of alphabetical)
      // Only apply when sort_by is not explicitly set
      if (!isset($params['filters']['sort_by'])) {
          $query->orderBy('ID', 'DESC');
      }
      return $query;
}, 10, 2);