If you've been using Shopify for some time, sometimes Shopify will have your emails set to "plain text" instead of HTML, which causes the emails to show the code instead of the customized design. Luckily, it's easily fixed:
Copy the new email code from my app
Open the email editing screen in Shopify
Click on the "Revert to default" button at the bottom of the screen
Once that is done, you can paste in the code from my app (replacing the code that is already there)
If you have any more troubles with it, click the little message icon in the bottom right corner of this page.