Mainly the shopping_cart class is used to process things.
The /includes/main_cart_actions.php is also used as a driver towards the cart class which does the actual adding.
However, it does it all based on whatever is being found in the $_POST vars ... ie: the form fields coming from the product page. If you've changed the product template in some way such that the fields being sent are different, then I'm not surprised that the add functionality is broken.




