Products - Add to cart I need some advice.

My client has a website with loads of products.  Now the client want the ability to add products to a cart, and instead of buying, they want the products to be mailed.

How can I acheive this.  I took a look at the commerce module.  This module is amazing.  I did some research and found this article saying I can achieve  what I want with the rules module.

This is awesome.  But drupal commerce has its own set op products that you need to add inside the store.  I need to use products already created. As well as add options such as colors and sizes that also needs to be displayed in Select lists. 

Is there a way that I can get this feature with existing products? Or just add the cart button to my nodes?

Thanks.
It depends on how your node 'products' are setup. In Drupal Commerce, 'products' are a fieldable Entity Type and each bundle of that entity type is a different product with different sets of fields. 

One possible solution would be to check out the Migrate and its ecosystem of other modules to see if there would be a way to convert your product Content Type to a Product Type.

Note that in the native Commerce workflow, you also need a Product Display node to act as a container for products to display them to the end user. Think of the Product entity as a single product option [in the example of a t-shirt it could be "X-Large Blue"] and the Product Display node is the wrapper that contains all options to display as a single "product" to the user.