The user manual documentation is included in the product download as a 132 page PDF. Installation was very easy. I setup my cart without needing the user manual because the admin section of CS-Cart is very good. Althought, I did do a couple of searches in the manual to locate some specific items.
Since you know PHP, you will find Smarty very easy to use. Smarty uses templates. When you open one, you will usually see html code with Smarty code between {}. When you run the cart, Smarty it is just like PHP code in HTML. As far as offering a free item if “Product A” is purchased, I haven’t seen this, but I would think it wouln’t be too difficult to add. Since I’m new to CS-Cart myself, anyone else know about Purchase exceptions?