How to add 'Pre-order Cart & Checkout label' to order confirmation email?

By default, Shopify doesn't include line item properties in the Order confirmation email template so we need to adjust it.
From your Shopify admin, go to Settings > Notifications.
In the "Email Templates" section, click New order.
You can add this code to the New order email template to include the 'Pre-order Cart & Checkout label':
{% unless line.properties == empty %}
  {% for property in line.properties %}
  {% assign first_character_in_key = property.first | truncate: 1, '' %}
  {% unless property.last == blank or first_character_in_key == '_' %}
    <span>{{ property.first }}: {{ property.last }}</span>
  {% endunless %}
  {% endfor %}
{% endunless %}
Add the above code just before:
{% if line.refunded_quantity > 0 %}
  <span>Refunded</span>
{% endif %}
	
So it looks like this: