Ran into an issue today that I thought I would share a fix for, having catalog and shopping cart pricing rules stack their discount on each other. What do I mean?
Say you have a catalog price rule that gives a 25% discount to all items in a certain category so a product with a base price of $20 would display with a discount to $15. Now, you have a coupon code for 30% off any purchase setup as a shopping cart price rule in the admin. What happens when a user adds this item to their cart and applies the coupon code? With default Magento it runs as follows:
More details :http://www.magentotips.info/modules/catalog-shopping-cart-pricing-rules-stacking/