Magento Shopping Cart Rule - Is this combination possible - magento

Question

Hoping some one can help me with a Magento Rule - is this rule even possible
I have numerous products of numerous sizes all part of the same category.
Each product regardless of size costs £3.98 - If a buyer buys 3 products of the same category regardless of the product or size they get it for £9.99. If they buy 4 products, they get 3 of them for 9.99 but pay full price for the 4th...Every group of 3 is £9.99
I have a rule created that seems to work perfect if a Customer buys 3 / 6 / 9 items of the same product and same size...However if they mix and match it doesn't work (though they are the same category)
The rule is:
IF ALL of these conditions are TRUE:
If total quantity equals or greater than 3 for a subselection of items in cart matching ALL of these conditions:
Category is 4
I have also set the Discount Qty Step to be 3
* UPDATE *
Thanks for your reply - I have tried to implement what you suggest and have got so far as to where I get the category id of the added products. I'm unsure how to set the price the previous products so it will be an automatically discounted price
$quote = Mage::getSingleton('checkout/session')->getQuote();
$cartItems = $quote->getAllVisibleItems();
$itemPrice = "3.33";
foreach ($cartItems as $items) {
$product = $items->getProduct();
$prodCats = $product->getCategoryIds();
if (in_array('4', $prodCats)) {
$itemQty = $items->getQty();
}
$totalItems += $itemQty;
}
So what I want to do is apply a discount for multiple of 3's for any product that has a category_id of 4...The price will be 3.33 instead of the normal 3.99

Answer 1

You need event - observer approach for this, that will give you the flexibility you need. You can build an observer that catches the add-to-cart event sales_quote_add_item and put your logic there.
Following code within your observer function will point you in the right direction:
// Get products in cart:
$quote = Mage::getSingleton('checkout/session')->getQuote();
$cartItems = $quote->getAllVisibleItems();
foreach ($cartItems as $item) {
$itemSku = $item->getSku();
$itemQty = $item->getQty();
}
// Added product:
$item = $observer->getEvent()->getQuoteItem();
$itemQty = $item->getQty();
$itemSku = $item->getSku();
// Change price of added product:
$item->setOriginalCustomPrice($newPrice);
Good luck!