Auto Complete Orders in WooCommerce

You can mark every order as completed automatically by adding the following code to your functions.php file. You can also change Completed to Processing to make every order processing automatically:

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

    $order = wc_get_order( $order_id );
    $order->update_status( 'completed' );
}
add_action( 'woocommerce_thankyou', 'custom_woocommerce_auto_complete_order' );

For more WooCommerce hooks, see Mastering WooCommerce Hooks.