Using Shopify metafields - custom fields in order notifications

Sample code

{% for line in line_items %} 
{{ line.product.metafields.namespace.key }} 
{% endfor %} 

Examples of Liquid drops to use in e.g. a Shopify notification template: 

{{ order.metafields.c_f.purchase_order_reference }}
{{ order.customer.metafields.c_f.id }}
{% for line in line_items %}
 {{ line.product.metafields.c_f.gift_option}}{{line.variant.metafields.c_f.washing_instructions }}
{% endfor %}

This example only applies to Shopify's internal notification templates. If you want to use the Order Printer app, metafields will not be available, so another approach is needed - please see this article from Shopify

You may find more detail about line item at https://shopify.dev/docs/themes/liquid/reference/objects/line_item