After 4 HARD months working with the SS scripts, let me throw my 2 cents in here. I have found, even with the "Clean" template, and now the "Simple" (new) template, that it is very hard to transpose this into an existing site template. I think a Smarty programmer may be able to do it, but I have had no luck. LOVE those blank white pages that come up!
However, this is how I've solved the problem. The recent new "Simple" template allowed me to do this, because it truly is a simple, clean template.
It sounds like you have an existing site, and you are wanting to add a store. Based on that, you can expect that your visitors will expect that the interface will look and feel the same. But, because this IS a store, I think everyone can live with it looking just a tiny bit different than the main pages in your site. The things that happen on category pages and detail pages are quite different from what happens on your primary site pages.
After 20 years on the computer (professional, education, personal) and some 14 years online, mostly in business capacity - and given my life's work even before Net.... the main thing that ties ANYTHING together is the VISUAL COMPONENT.
So, here is what is needed. Tie your store and all of its pages to your main site... with the primary visual elements of your primary site. This can include backgrounds (this turned out to be very tricky however, so I opt for a nice white background... but I will continue to explore this), or headers, or Google leaderboards, etc.
With the Simple template, you are only dealing with a few GIF images. These can be taken into Photoshop, set at RGB for editing, and then you change the colors of the Menu images, to match the main colors on your primary site. Resave them as GIFs and upload. Even with a plain white background and the newly colored Menu images, the Store begins to take on the feeling of your site.
I then added elements .... I recreated a simpler version of my primary header, and played with the CSS to achieve the same look and feel for the links. The same in CSS for the same colored scrollbar (yes this only works in IE but I design primarily for IE). The site title at the top makes it very clear that this is the Store, as opposed to the main site. And same with the links - they immediately show the link back to the primary site's home page. I follow through with design elements in the footer, which tie the Store VISUALLY back to the home page for the domain.
The design for my primary domain has been ongoing, when I could fit it in, for these 4 months. But it was only 4 days ago on the 22nd, that I implemented the SS Store. It has been very quick and easy. But then, I've been HARD at this for 4 months, in learning the SS scripts and how Smarty works.
The site:
1WebDiva.comNote the custom menu/navigation scheme. The store link is right at the top.
If you feel that this may be a solution for you, as well - I can help. My store has only 3 categories, with about another 10 subcategories. The current Sitemap is almost 200KB and lists somewhere between 800 and 1000 listings.
Again, this solution will take about two days. Perhaps a bit more, as I will need to familiarize myself with your niche and content.
My usual rate is $25/hr. However, I will use two days' time and create your Storestacker store, customized to your current site's graphics, for $195.00. This reflects appx. 15 hours work, at appx. $13/hr. This may involve more like 20 hours to complete, but I stick with the price. I am only doing this for my Storestacker peers.... it's been a very difficult 4 months of learning all of this. And I am aware that many of you are having trouble.