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<?php
//WC < 2.5.x
//* Oculta resto de métodos de envío cuando envío gratuito está disponible
add_filter( 'woocommerce_package_rates', 'mostrar_solo_envio_gratis', 10, 2 );
function mostrar_solo_envio_gratis( $rates, $package ) {
    global $woocommerce;
    
    if ( isset( $rates[ 'free_shipping' ] ) ) {

        // Sólo muestra la opción de envío gratuito
        $free_shipping          = $rates[ 'free_shipping' ];
        $rates                  = array();
        $rates[ 'free_shipping' ] = $free_shipping;
    }

    return $rates;
}

//WC >=2.6

/**
 * Hide shipping rates when free shipping is available.
 * Updated to support WooCommerce 2.6 Shipping Zones.
 *
 * @param array $rates Array of rates found for the package.
 * @return array
 */
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;
}
add_filter( 'woocommerce_package_rates', 'my_hide_shipping_when_free_is_available', 100 );