title: I want to add a custom email variable to Shopify Order Notification
body: <p>Could anyone tell me how to add a custom email variable to a shopify order notification?</p>
  
  <p>For example, by default the order notification already contains the following:</p>
  
  <pre><code>  Shipping address
      {{ shipping_address.name }}
      {{ shipping_address.street }}
      {{ shipping_address.city }}, {{ shipping_address.province }}  {{ shipping_address.zip }}
      {{ shipping_address.country }}{% endif %}
  </code></pre>
  
  <p>These variables then translate into the actual shipping address of the customer in particular.
  Now I want to add a line of information from a third party source into this order notification, how would I go about this?
  I need a license key put in the Shopify order notification e-mail but this license key comes from a service outside of Shopify, is there any way I could make this happen one way or another?</p>