I'm trying to setup a customized cart rule, so that when an item in the shopping cart has subscriptions enabled, that particular product has free shipping.
I'm new to Magento and managed to modify the rules management in the admin panel to add the "is_subscription_enable" rule by extending the conditions model.
What I haven't found yet, is where exactly does Magento processes and applies the shopping cart rules. Is it when items get added to the shopping cart? When building the checkout page? or is every rule applied individually and it modifies the product properties every time a new rules is added, modified or deleted?
What would be the best approach to achieve this task?