Need help with shopping cart script

I am working on a site that another developer started. He used Codeigniter, and stopped before adding a shopping cart. There is no way to order items from the site. I was wondering if it would be better to create a customized shopping cart, or to use something like zencart and customize it to match the site. Keep in mind that I am a newbie at php and codeigniter. Is there a custom shopping cart script out there that I might be able to adjust to fit my needs?

The answer to the question is predicated on a few factors. You need to define the scope of your ecommerce (are you selling one product or many), what features you require (both for the end-user and for your admins), and how closely does your ecommerce experience integrate with your current site. If you want a lot of features (like user accounts, product search, gift certificates, recommend to a friend, sales reports, etc) and you don't mind
if the shopping part of your site looks and behaves a little different than your main CI-based site, then using a third-party solution is the way to go. I see this with lots of sites where the 'store' is actually a subsite -often designated with its own subdomain (e.g. 'shopping.mysite. com'). The user experience is different than the main site (although most branding elements are present) but that's acceptable since the mission of the user
is now shopping. You are trading off strong integration with quick deployment of a testing suite of functionality.

However, if your main site is part of the product's marketing and it needs to have a cart/purchase functionality strongly tied to it, you will need to build the cart using CI. You can jump start this process by finding a very basic shopping cart class (which you will treat as a custom Library in CI). The CI documentation detail custom Libraries. The CI forums have posts about how to customize classes into CI-style custom Libraries. You'll also
need to integrate your payment gateway. That ought to be much easier though.