AgoraCart is free. It can be integrated with PayPal so there is no additional need for sometimes costly gateways.
It can be blended into your current website without too much difficulty. That would of course depend on the amount of inventory already posted.
Since it can track inventory and can be integrated with both your website and PayPal... this feature rich shopping cart system is the most powerful and one of the easiest to install. Simply download it from
AgoraCart.com and then follow the installation instruction that come packaged with it.
You can edit the headers and footers to blend into the site quite nicely but that is possibly the most trtouble. If you can do that with some ease... the rest is super easy.