If you want to easily integrate a shopping cart and online store in to the existing webpages of your website, without having to change your website CMS or existing design, then you could look at hosted cart store add-in solution to integrate straight into your existing website CMS.
A hosted cart store add-in makes it easy to copy and paste store buttons in to your webpages so the store integrates straight in to your existing website. It also make it straight forward to connect to your secure card payment processor for taking customer payment for an order in to your account, e.g. PayPal as a payment processor.
A hosted cart store add-in solution will mean you will have no software or database to install and manage on your own servers as this is all managed for you. It also means you'll automatically gain the latest features and enhancements to the shopping cart software without having to manually upgrade the shopping cart software yourself.
If you wanted any assistance with what you are planning to do then I would be happy for your to send me a Private Message where I will be able to help you out.