add_filter('woocommerce_package_rates', function($rates, $package) {
    foreach ($rates as $rate_id => $rate) {
        if ('flat_rate' === $rate->method_id) {
            $item_count = WC()->cart->get_cart_contents_count();
            
            if ($item_count == 1) {
                $rates[$rate_id]->cost = 10; // Flat price for 1 item
            } elseif ($item_count >= 2) {
                $rates[$rate_id]->cost = 15; // Flat price for 2 or more items
            }
        }
    }

    return $rates;
}, 10, 2);