Exibir frete grátis em vez do preço zerado no frete

functions.php

add_filter( 'woocommerce_cart_shipping_method_full_label', 'fa_add_free_shipping_label', 10, 2 );
function fa_add_free_shipping_label( $label, $method ) {
  if ( $method->get_cost() == 0 ) {
    $label  = str_replace( wc_price( 0 ), '', $label );
    $label .= ' Frete grátis';
  }
  return $label;
}