So basically, I've never found an ecommerce solution yet that just works.
And I've tried a lot of them.
They often require the cusom creation of templates, as the shops css and the theme css never seem to play right. Everything always looks pretty crappy for me.
Now that being said, the easiest I've dealt with is eshop. Well, the easiest in terms of appearance. It just works through the normal post interface. I set up a custom post type and used it for my shop. I like it, but wasn't 100% sold on the back end. Some things were a bit confusing to me, but I'm not an e commerce expert.
I've recently began experimenting with jigoshop which has an easier to understand backend, but I'm having trouble with appearance. This plugin sets up its own custom post types for you, and sets up pages for your cart, and shop, etc. It seems pretty solid and thorough, but again...appearance on the front-end. It allows you to turn off the css for the plugin, but my shop seems to be busting right out of my theme.
So basically what it comes down to is figuring out the mechanism for your ecommerce plugin. Is it custom post type based? If so it's pretty easy to creat custom templates. But to get what you are after, you'll need to use a combination of templates most likely, maybe custom queries for displaying the right number of products, and custom css.
Sorry I can't be more specific. I've just found that every plugin/theme combination is different. And every one of the ecommerce plugins requires a fair amount of customizing through the theme to get right.