Can OC drop into an existing site inside a div

Hi all,
I'm new here as well as to shopping carts. We are looking to drop in a cart to an existing site, just using the products, shopping cart and checkout pages. We already have the layout (I believe that is considered the theme or template in the cart world), menus and hyperlinks and the customer wants to maintain the same for all pages. Is this possible with OC or with any shopping cart application or should we consider writing our own?
We're looking for the best direction to take for this and other sites.
Initially, the customer wants to put up 6 products with the ability to expand in the future, based on how it progresses over time.
Thank you very much.

I would think it would be easier to just skin oc to look like the other site. Open cart is very easy to theme.

Thanks jayman. I appreciate your response.
What area's do I need to be concerned about to use the backend logic, create a new module (ipCommerce) and allow OC to plug into an existing site?