// Rename the flat rate shipping label when the cost is $0
function wc_ninja_change_flat_rate_label( $label, $method ) {
	if ( 'flat_rate' === $method->method_id && $method->cost == 0 ) {
		$label = "Free Shipping";
	}

	return $label;
}
add_filter( 'woocommerce_cart_shipping_method_full_label', 'wc_ninja_change_flat_rate_label', 10, 2 );