Simple Magento that hooks onto the Add to Cart event. Can be used to redirect somewhere, but we use to apply relational logic for our products. Example: If product has attribute "length" over 24", then add an extra packaging fee to the cart. Updated version includes more complex business logic to add packaging fee for every 8 items for specific …

config.xml

<!-- File location: /app/code/local/Mbs/Addtocart/etc/config.xml -->
<?xml version="1.0"?>
<config>
	<modules>
		<Mbs_Addtocartredirect>
			<version>1.0</version>
		</Mbs_Addtocartredirect>
	</modules>

	<global>
	    <events>
	        <checkout_cart_add_product_complete>
	            <observers>
	                <addtocartredirect>
	                    <type>singleton</type>
	                    <class>Mbs_Addtocartredirect_Model_Observer</class>
	                    <method>addToCartRedirect</method>
	                </addtocartredirect>
	            </observers>
	        </checkout_cart_add_product_complete>
	    </events>
	</global>
</config>

Observer.php

<?php
  // File location: /app/code/local/Mbs/Addtocart/Model/Observer.php
	class Mbs_Addtocartredirect_Model_Observer {
		public function addToCartRedirect($observer) {

			$event = $observer->getEvent();
			$product = $event->getProduct();
			$productId = $product->getId();

			// Add hand packing fee if product fits criteria
			// Checks if hand pack fee is required & if there is already a fee applied in the cart
			$quote = Mage::getSingleton('checkout/session')->getQuote();
			$handPackProduct = Mage::getModel('catalog/product')->load(1621);
			$needsHandPack   = false;    // Hand packing fee charged for objects that require special packaging
			$hasHandPack     = false;	// Does customer have Hand packing fee in cart already?
			$handPackQty     = 0;		// How many HandPack fees are charged?
			$handPackQtyNeed = 0;		// How many should we have?
			$multPackArray   = array("ZCP1-3","ZCP1-6","ZCP2-3","ZCP2-6","ZCP3-3","ZCP3-6","SRG1900-1","SRG1900-2","SRG1900-3","SRG1900-4",); // What products require one HandPack fee per each item?

			foreach ($quote->getAllItems() as $item) {
				$storeId       = 1;      	// Mbs Standoffs store code
				$attributeId   = 'height';	// Height (BE) attribute code
				$productId     = $item->getProduct()->getId();
				$productSku    = $item->getProduct()->getSku();
				$productLength = Mage::getResourceModel('catalog/product')->getAttributeRawValue($productId, $attributeId, $storeId);

				// If length is more than 24", product requires a HandPack fee
				if($productLength >= 24) {
					$needsHandPack = true;
					$handPackQtyNeed += 1;
				}

				// If product has a HandPack fee already, we want to know how much
				if($productId == 1621)  {
					$hasHandPack = true;
					$handPackQty = $item->getQty();
				}
								
				// If product is one of a select few, they need a HandPack fee for each 8 items.
				// This keeps track of how many we should have
				if( in_array($productSku, $multPackArray) ) {
					$handPackQtyNeed += $item->getQty() - 1;
				}
			}


			// If we need more than we have already, add the difference to the cart
			if($handPackQtyNeed > $handPackQty) {
				$handPackQtyNeed = ceil( ($handPackQtyNeed - $handPackQty) / 8 );
				$quote->addProduct($handPackProduct, $handPackQtyNeed);
			} 
			// Else If 
			elseif($needsHandPack && !$hasHandPack) {
		    	$quote->addProduct($product,1);
			}

		    $quote->collectTotals()->save();
			// $goToUrl = Mage::getBaseUrl()."accessories_section?id=".$productId."&sec=grid";
			// Mage::app()->getResponse()->setRedirect($goToUrl)->sendResponse();
			// exit;

		}
	}

?>