I have this working on my site.
But it only seems to work in Safari for me.
What I did was add the javascript to the viewinvoice.tpl file in the templates/default folder. Place the code right after the body tag.
</body>
<script src="https://www.2checkout.com/static/checkout/javascript/direct.min.js"></script>
</html>
If anyone has a clue why it is not working in chrome, please let me know.