If your site is in html, it should be easy to migrate current look and feel in to a Drupal theme, then you can use the Drupal modules to add ecommerce functionality to your Drupal site. Other alternatives include using an external cart (costs $) or using paypal based shoppping cart (works if you have a handful of products - is time consuming to manage.

Go through the videos at http://www.seoecom.com/cms to learn how to create full Drupal ecommerce site or skip through just the modules you need from http://www.seoecom.com/sitemap