Order Printer - MetaFields in Template by Daniel Pape

Greetings all. I use the Order Printer app for generating invoices to accompany my international order shipments. I am using the metafields API to store some additional information with each of my products. I would like to call this information in my Order Printer template.

Here is my code:

Made in: {{ product.metafields.customs.country }}
Materials: {{ product.metafields.customs.materials }}

Here is the context:

{% for line_item in line_items %}<tr><td>{{ line_item.quantity }} x</td><td><b>{{ line_item.title }}</b><br />
            Made in: {{ product.metafields.customs.country }} <br />
            Materials: {{ product.metafields.customs.materials }}</td><td>{{ line_item.price | money }}</td></tr>
{% endfor %}

This is not working for me. I have played around with other supported Liquid tags, and can not get any product information to display in my invoice template. Am I missing some sort of product call to connect my line_item(s) back to their product information?

Thanks for the help.