file.php

<?php
/**
 * Display (Free) when shipping method does not charge any cost.
 *
 * @param  String           $label  Shipping method label.
 * @param  WC_Shipping_Rate $method Shipping method data.
 * @return string
 */
function serdika_free_shipping_label( $label, $method ) {
	// Only apply when is free and not using the free shipping method.
	if ( '0.00' === $method->cost && 'free_shipping' !== $method->method_id ) {
		$label = sprintf( __( '%s: (Free)', 'text-domain' ), $label );
	}

	return $label;
}

add_filter( 'woocommerce_cart_shipping_method_full_label', 'serdika_free_shipping_label', 10, 2 );