This seems to be way more convoluted than it needs to be, but I’m not sure how much of that is that it’s actually convoluted, how much is that Woo’s documentation sucks, and how much is that everyone else’s tutorial on it is tl;dr.

Anyway… I just wanted to do something fairly simple. I want to have each product’s short description get sent into the order data. This is a specific use case with a client who’s syncing data over the REST API with an external system, and we’re shoehorning data into the short description that maybe could go somewhere else. The point is, use your imagination as to how this might be useful to you.

I’m stripping out a lot of the other details. All I want is a way to a) add the data to the item in the cart, and b) carry that data over into the order item meta data in the database. You may need or want more, but this will get you started.

// Add custom order item meta data to cart
add_filter('woocommerce_add_cart_item_data', function($cart_item, $product_id) {
  if (!isset($cart_item['short_description'])) {
    if ($product = wc_get_product($product_id)) {
      $cart_item['short_description'] = $product->get_short_description();
    }
  }
  return $cart_item;
}, 10, 2);

// Add custom order item data from the cart into the order
add_action('woocommerce_checkout_create_order_line_item', function($item, $cart_item_key, $values, $order) {
  if  (isset($values['short_description'])) {
    $item->add_meta_data('Short Description', $values['short_description'], true);
  }
}, 10, 4);

This is a major distillation of stuff I found in these two tutorials: How to Add a Customizable Field to a WooCommerce Product and Add Custom Cart Item Data in WooCommerce.