I found that inserting ZenCart into Joomla is near impossible and ultimately does not work well. (The Joomla wrapper function is inadequate for this.) Finding ZenCart and Joomla templates that were similar enough to begin with, and then attempting to synch them, takes an ENORMOUS amount of effort. I was considering building my ZenCart template from scratch to match my Joomla site, when I ran across the Sanguis Easy Embed Template contribution for ZenCart. Here's a link to the Easy Embed Template thread: http://www.zen-cart.com/forum/showthread.php?t=42105
The Easy Embed template is a ZenCart template that allows you to create an HTML wrapper to insert ZenCart. It is the ZenCart template for folks who have an existing website, but would like to add ZenCart while still using their existing site design. It seemed like the way to go if I wanted to use Joomla with ZenCart.
Here's how I implemented this:
I used plain old HTML to make this happen, but I suppose a real PHP programmer could figure out a way to call the Joomla code from the ZenCart embedded template. I may one day look into doing just this and having a developer create this code for me. So for now my simple HTML solution works just fine.
ZenCart has been a MUCH better solution for my clients, and I will continue to offer it as an e-commerce solution. It has a rich feature set which rivals some of the commercial carts available.
See also: Joomla ZenCart extensions