WooCommerce - Make VAT number optional if cart total is zero

/**
 * Customisation for the WooCommerce EU VAT Assistant.
 * Makes the VAT Number optional when the cart total is zero, and required when the total is
 * greater than zero. For more details, please refer to the original article:
 * https://aelia.co/2017/01/27/woocommerce-eu-vat-assistant-vat-number-optional-cart-total-zero/
 *
 * Need help customising the code for your need? Hire us on Codeable: http://bit.ly/codeable_aelia
 *
 * @author Aelia
 * @link https://aelia.co/2017/01/27/woocommerce-eu-vat-assistant-vat-number-optional-cart-total-zero/
 * @license GPLv3
 */

/* This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 *
 * We (Aelia) won't be able to provide free assistance or support
 * for this code, nor can we be held responsible for any issue,
 * downtime or damage caused by its implementation.
 */

/**
 * Adds the cart total to the fragments returned as a response to the Ajax
 * requests on the checkout page.
 *
 * @param array fragments The fragments returned as a response.
 * @return array
 */
add_filter('woocommerce_update_order_review_fragments', function($fragments) {
  $fragments['_cart_total'] = WC()->cart->total;
  return $fragments;
});

/**
 * Adds a script to the checkout page, to make the VAT number required or not
 * required, depending on the cart total.
 */
add_action('wp_footer', function() {
  // We need to render our script only on the checkout page
  if(!is_checkout()) {
    return;
  }
  ?>
  <script>
  jQuery(document).ready(function($) {
    // Run the script every time the checkout form is updated. This will
    // allow us to check if the total changed
    $(document.body).on('updated_checkout', function(ev, data) {
      if(!data['fragments'] || !data['fragments'].hasOwnProperty('_cart_total')) {
        return;
      }

      var cart_total = parseFloat(data['fragments']['_cart_total']);
      var vat_number_required = (cart_total > 0);

      var $eu_vat_number = $('#woocommerce_eu_vat_number');
      $eu_vat_number.toggle(vat_number_required);
      $eu_vat_number.find('.form-row').toggleClass('validate-required', vat_number_required);
    });
  })
  </script>
  <?php
});

/**
 * Sets the VAT number field as "not required" when the cart total is zero (or
 * less; which should never happen, but better to cover that case).
 *
 * @param bool is_vat_number_required Indicates if the VAT number is required.
 * @param string country The billing country selected at checkout.
 * @return bool
 */
add_filter('wc_aelia_euva_order_is_eu_vat_number_required', function($is_vat_number_required, $country) {
  // Make VAT number "not required" if the cart total is zero
  if(WC()->cart->total <= 0) {
    $is_vat_number_required = false;
  }

  return $is_vat_number_required;
}, 10, 2);