Show the Word "FREE" When Shipping is $0 from Third Party Plugin

wc-show-free-shipping-text.php

/**
 * Show the word "FREE" when shipping is free from third party shipping plugins
 * Code snippets should be added to the functions.php file of your child theme
 * (Updated for WooCommerce 3.5)
 *
 * @return string
 */
add_filter( 'woocommerce_cart_shipping_method_full_label', function( $label, $method ) {
    // only update for Free shipping methods
    $has_cost  = 0 < $method->cost;
    if ( ! $has_cost ) {
        $label = $method->get_label() . ': FREE';
    }

    return $label;
}, 10, 2 );