There are two steps to correctly install Tend on Shopify:
Go to the theme editor at https://myshopify.com/admin/themes. Then select the Edit HTML/CSS option.
Within the theme.liquid file (this file may be different depending on your theme), copy/paste the Tracking Code from your Tend account, just above the </head>
tag.
Within Shopify, go to Settings > Checkout. There, scroll down to "Additional Scripts." Here, place the Tracking Code from your Tend Account, along with the TendID script for shopify. Should end up with something like...
<!-- Tend Code -->
<script type="text/javascript">
window.tendID = {
email: "{{ checkout.email }}"
};
</script>
<script type="text/javascript" src="https://track.tend.io/js/track/v2.minified.js"></script>
Since Shopify does not allow you to install a script on the checkout pages, TendID is used to "push" the customer's email into Tend.
And that's it! Of course, let us know if we can help!