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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:

Example

Categories:

• Shoes (3% discount)




• Coats (2% discount)




• Jackets (2% discount)




• Sale (Exempt from discount)




• Accessories (5% discount)

Scenario:

• 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.

The individual sales rules are possible, so you can create a cart rule that will give a percentage for any order containing items bought from a given category. You can then create other rules for other categories. So each of them could cumulatively give e.g. 2% off the order.

However, those rules are not aware of each other. So they do not know what the current discount is (from the other rules) and cannot then have the 'if' to decide whether a further discount is possible.

As for the product specific discount, you can do that one with a rule or a just a special price on the product.

If you want to do this with a module then the easy bit is coding the rules that you want. Quite a few other changes are needed. There may be a need to update templates (to show your extra line item), some xml for 'totals_sort' order - it goes on.

I also feel that your original sales idea might actually be confusing for customers. Sometimes the spend X get Y rules need to be easy for people to understand.