By default, the Print Invoice and Packing Slip plugin for WooCommerce plugin allows you to display meta data in a pre-defined place holders. At times, store owners may require custom placeholders for meta data. Simply insert the below given code snippet in to function.php of active child theme to have custom placeholders for WooCommerce labels.
In the code snippet above, you can replace the _meta_key_1, _meta_key_2 and replace with the meta key of the actual meta in the post meta that you want to add to the labels. Also, make sure to copy the place holders in the template HTML code.