free-shipping-option.php

<?php 

add_filter( 'woocommerce_package_rates', 'my_hide_shipping_when_free_is_available' );

function my_hide_shipping_when_free_is_available( $rates ) {
        $free = array();
        foreach( $rates as $rate_id => $rate ) {
          if( 'free_shipping' === $rate->method_id ) {
                $free[ $rate_id ] = $rate;
                break;
          }
        }

        return ! empty( $free ) ? $free : $rates;
}


?>