Add a WooCommerce custom order action

wc-custom-order-action-sample.php

<?php // only copy if needed

/**
 * Add a custom action to order actions select box on edit order page
 * Only added for paid orders that haven't fired this action yet
 *
 * @param array $actions order actions array to display
 * @return array - updated actions
 */
function sv_wc_add_order_meta_box_action( $actions ) {
	global $theorder;

	// bail if the order has been paid for or this action has been run
	if ( ! $theorder->is_paid() || get_post_meta( $theorder->id, '_wc_order_marked_printed_for_packaging', true ) ) {
		return $actions;
	}

	// add "mark printed" custom action
	$actions['wc_custom_order_action'] = __( 'Mark as printed for packaging', 'my-textdomain' );
	return $actions;
}
add_action( 'woocommerce_order_actions', 'sv_wc_add_order_meta_box_action' );


/**
 * Add an order note when custom action is clicked
 * Add a flag on the order to show it's been run
 *
 * @param \WC_Order $order
 */
function sv_wc_process_order_meta_box_action( $order ) {

	// add the order note
	$message = sprintf( __( 'Order information printed by %s for packaging.', 'my-textdomain' ), wp_get_current_user()->display_name );
	$order->add_order_note( $message );

	// add the flag so this action won't be shown again
	update_post_meta( $order->id, '_wc_order_marked_printed_for_packaging', 'yes' );
}
add_action( 'woocommerce_order_action_wc_custom_order_action', 'sv_wc_process_order_meta_box_action' );

On sv_wc_process_order_meta_box_action function, how possible display a message box to admin ?

Currently code uses update_post_meta function and add_order_note method and not display any message to admin.

thanks!