/**
 * Auto Complete all WooCommerce orders.
 */
add_action('woocommerce_thankyou', 'custom_woocommerce_auto_complete_order');
function custom_woocommerce_auto_complete_order($order_id)
{
    if (!$order_id) {
        return;
    }

    $order = wc_get_order($order_id);
    $order->update_status('completed');
}