/**
 * @snippet       Hide free shipping method and set shipping costs of other methods to 0 and show message when reach free shipping amount for cart
 * @author        Dexter
 * @testedwith    WooCommerce 3.5.5
 */
add_filter( 'woocommerce_package_rates', 'dexter_apply_free_shipping_to_all_methods', 10, 2 );
function dexter_apply_free_shipping_to_all_methods( $rates, $package ) {

  //jeżeli klient ma darmową dostawę to…
  if( isset( $rates['free_shipping:21'] ) ) {
    //ukrywamy darmową dostawę. Ma to sens bo…
    unset( $rates['free_shipping:21'] );
    unset( $rates['flat_rate:17']);
    //wszystkie dostawy stają się za kwotę 0zł
    foreach( $rates as $rate_key => $rate ) {
                // Append rate label titles (free)
                $rates[$rate_key]->label .= ': ' . __('0,00 zł', 'woocommerce');

                // Set rate cost
                $rates[$rate_key]->cost = 0;

                // Set taxes rate cost (if enabled)
                $taxes = array();
                foreach ($rates[$rate_key]->taxes as $key => $tax){
                    if( $rates[$rate_key]->taxes[$key] > 0 )
                        $taxes[$key] = 0;
                }
                $rates[$rate_key]->taxes = $taxes;
    }
  }
  return $rates;
}