Free Shipping in WooCommerce https://wpsites.net/wordpress-tutorials/code-to-add-free-shipping-minimum-in-woocommerce/

free-shipping-woocommerce.php

add_filter('woocommerce_package_rates', 'custom_woocommerce_available_shipping_methods', 100);
function custom_woocommerce_available_shipping_methods($rates) {
    $free_shipping_minimum = 100; // Set your minimum cart total for free shipping here
    $cart_total = WC()->cart->get_subtotal();

    if ($cart_total >= $free_shipping_minimum) {
        foreach ($rates as $rate_id => $rate) {
            if ('free_shipping' === $rate->method_id) {
                $free_shipping_rate = $rates[$rate_id];
                $rates = array($rate_id => $free_shipping_rate);
                break;
            }
        }
    }
    
    return $rates;
}