Integrate shopping cart into existing PHP site
So this is the first time I've worked on an eCommerce site, and I'm a little in the dark about integration.
The client has chosen CardSave as the gateway. Now the CardSave system wants to know which Shopping Cart will be used. So, I'm going with TomatoCart, mainly because the hosting package has an installer script for it.
But, here's my main issue, I've already built the site (pretty much), using PHP and MySQL. All the documentation for TomatoCart seems to suggest that I chose a template and then add products to that.
The site's design has already been done and sent to me, and I have created the site form scratch to these specs, including a CMS to add products to a MySQL database.
Am I completely screwed here, or just misunderstanding how Open Source shopping carts work?