WooCommerce: Replace free shipping label

functions.php

<?php
// Paste everything below this line to your child-theme's functions.php file.

// Change the free shipping label to 'Free shipping'.
add_filter('woocommerce_cart_shipping_method_full_label', static function ($label, $method) {
    if ($method->cost == 0) {
        $label = 'Free shipping';
    }
    return $label;
}, 10, 2);