/* Sort WooCommerce shipping rates from lowest to highest cost.
Add the below code to your functions.php file. */

add_filter( 'woocommerce_package_rates' , 'rd_sort_shipping_methods', 10, 2 );

function rd_sort_shipping_methods( $rates, $package ) {

    //Check to see if there are any rates
    if ( empty( $rates ) ) return;

    //Get a list of rates
    $temp = array();
    foreach( $rates as $rate ) {
        $temp[] = $rate->cost;
    }

    //Sort the rates based on cost
    array_multisort( $temp, $rates );

    return $rates;

}