Ray will certainly be better for you in answering the question but I thought I can at least share what the reason for my problem was.
In my case the problem was that the jquery.js was referenced and loaded twice and after removing one of the references it solved my problem.
The problem occurred after nesting the checkout.php pages into a template. The original template already had a reference to load the jquery.js in the footer such as this:
after applying the checkout wizard in eCart, another reference to load the jquery.js was added to the header of the page such as this:
After adding the code for the checkout.php page to the template = the checkout.php page now had 2 references to load jquery.js wich caused the problem.
Solution was to simply remove the reference line to the jquery.js in the footer:
I hope this helps and good luck.