I have a specific type of shopping cart rule I'd like to create. You can think of it as a 'variety discount'; the more products from different categories the customer has in their basket, the greater a discount they get from their order:
Shoes (3% discount)
Coats (2% discount)
Jackets (2% discount)
Sale (Exempt from discount)
Accessories (5% discount)
A customer adds a product to the (empty) cart from category Shoes worth £40, this adds a 3% discount to the cart total, resulting in £38.80 (40 - 3%)
Then a product from the Coats category worth £200 is added to the cart. The total discount increases to 5% for the cart total, resulting in £228 (240 - 5%)
A second product from shoes is added worth £30. As the price rule has already been applied, the discount isn't affected, resulting in a total of £256.50 (270 - 5%)
A sale item is added to the cart worth £10, at a reduced price of £8. This is exempt from the deduction for the other products, giving a total of £264.5.
The Shopping Cart Price Rule interface allows you to discount a fixed amount from the cart, and the alternative is applying a percentage amount to items that match the conditions set.
Is the described scenario above possible without writing a custom extension? If not some advice on where to get started would be appreciated as Magento is still a little alien to me.