update-cart-meta-data.php

<?php
/**
 * Update cart item notes
 */
function prefix_update_cart_notes() {
  // Do a nonce check
	if( ! isset( $_POST['security'] ) || ! wp_verify_nonce( $_POST['security'], 'woocommerce-cart' ) ) {
		wp_send_json( array( 'nonce_fail' => 1 ) );
		exit;
	}
	// Save the notes to the cart meta
	$cart = WC()->cart->cart_contents;
	$cart_id = $_POST['cart_id'];
	$notes = $_POST['notes'];
	$cart_item = $cart[$cart_id];
	$cart_item['notes'] = $notes;
	WC()->cart->cart_contents[$cart_id] = $cart_item;
	WC()->cart->set_session();
	wp_send_json( array( 'success' => 1 ) );
	exit;
}
add_action( 'wp_ajax_prefix_update_cart_notes', 'prefix_update_cart_notes' );
add_action( 'wp_ajax_nopriv_prefix_update_cart_notes', 'prefix_update_cart_notes' );

function prefix_checkout_create_order_line_item( $item, $cart_item_key, $values, $order ) {
	foreach( $item as $cart_item_key=>$cart_item ) {
		if( isset( $cart_item['notes'] ) ) {
			$item->add_meta_data( 'notes', $cart_item['notes'], true );
		}
	}
}
add_action( 'woocommerce_checkout_create_order_line_item', 'prefix_checkout_create_order_line_item', 10, 4 );

thanks, very helpful!