WooCommerce - Apply free shipping to all shipping methods

woocommerce-free-shipping.php

<?php
/**
 * Plugin name: WooCommerce - Apply free shipping to all shipping methods
 * Plugin URI: https://gist.github.com/anttiviljami/ad46752d23d622c07f82
 * Description: If Free Shipping is available, make other shipping options cost 0 and disable the 'Free Shipping' method
 * Version: 1.0
 * Author: anttiviljami
 * Author: https://github.com/anttiviljami
 * License: GPLv3
 */

add_filter( 'woocommerce_package_rates', 'wc_apply_free_shipping_to_all_methods', 10, 2 );
function wc_apply_free_shipping_to_all_methods( $rates, $package ) {
  if( isset( $rates['free_shipping'] ) ) { 
    unset( $rates['free_shipping'] );
    foreach( $rates as $rate ) { 
      $rate->cost = 0;
      $rate->taxes = array();
    }   
  }
  return $rates;
}