* Auto Complete all WooCommerce virtual orders.
 * 
 * @param  int  $order_id The order ID to check
 * @return void
 */
function custom_woocommerce_auto_complete_virtual_orders( $order_id ) {

    // if there is no order id, exit
    if ( ! $order_id ) {
        return;
    }

    // No updated status for orders delivered with Bank wire, Cash on delivery and Cheque payment methods.
    if ( in_array( $order->get_payment_method(), array( 'bacs', 'cod', 'cheque', '' ) ) ) {
        return;
    } 

    // get the order and its exit
    $order = wc_get_order( $order_id );
    $items = $order->get_items();

    // if there are no items, exit
    if ( 0 >= count( $items ) ) {
        return;
    }

    // go through each item
    foreach ( $items as $item ) {

        // if it is a variation
        if ( '0' != $item['variation_id'] ) {

            // make a product based upon variation
            $product = new WC_Product( $item['variation_id'] );

        } else {

            // else make a product off of the product id
            $product = new WC_Product( $item['product_id'] );

        }

        // if the product isn't virtual, exit
        if ( ! $product->is_virtual() ) {
            return;
        }
    }

    /*
     * If we made it this far, then all of our items are virual
     * We set the order to completed.
     */
    $order->update_status( 'completed' );
}
add_action( 'woocommerce_thankyou', 'custom_woocommerce_auto_complete_virtual_orders' );