For Expresso Store v1. Add a $10 discount for every item in the cart which for three or more items. In Store v2 this is now included natively.

ext.store_discount_batch.php

<?php  if ( ! defined('BASEPATH')) exit('No direct script access allowed');
 
/**
 * Store Discount Extension
 *
 * @package     ExpressionEngine
 * @subpackage  Addons
 * @category    Extension
 * @author      Ian Young
 * @link        http://
 */
 
class Store_discount_ext {
 
    public $settings        = array();
    public $description     = 'Apply bath quantity-based discounts to Exp:resso Store';
    public $docs_url        = 'http://';
    public $name            = 'Store Batch Discount';
    public $settings_exist  = 'n';
    public $version         = '1.0';
 
    private $EE;
 
    /**
     * Constructor
     *
     * @param   mixed   Settings array or empty string if none exist.
     */
    public function __construct($settings = '')
    {
        $this->EE =& get_instance();
        $this->settings = $settings;
    }
 
    // ----------------------------------------------------------------------
 
    /**
     * Activate Extension
     *
     * This function enters the extension into the exp_extensions table
     *
     * @see http://codeigniter.com/user_guide/database/index.html for
     * more information on the db class.
     *
     * @return void
     */
    public function activate_extension()
    {
        // Setup custom settings in this array.
        $this->settings = array();
 
        // Add hooks
        $data = array(
            'class'     => __CLASS__,
            'method'    => 'update_cart',
            'hook'      => 'store_cart_update_end',
            'settings'  => serialize($this->settings),
            'priority'  => 10,
            'version'   => $this->version,
            'enabled'   => 'y'
        );
 
        $this->EE->db->insert('extensions', $data);
    }
 
    // ----------------------------------------------------------------------
 
    /**
     * Disable Extension
     *
     * This method removes information from the exp_extensions table
     *
     * @return void
     */
    function disable_extension()
    {
        $this->EE->db->where('class', __CLASS__);
        $this->EE->db->delete('extensions');
    }
 
    // ----------------------------------------------------------------------
 
    /**
     * Update Extension
     *
     * This function performs any necessary db updates when the extension
     * page is visited
     *
     * @return  mixed   void on update / false if none
     */
    function update_extension($current = '')
    {
        if ($current == '' OR $current == $this->version)
        {
            return FALSE;
        }
 
        $this->EE->db->where('class', __CLASS__);
        $this->EE->db->update(
                    'extensions',
                    array('version' => $this->version)
        );
    }
 
    // ----------------------------------------------------------------------
 
    /**
     * Responds to the store_cart_update_end hook in Exp:resso Store
     * to apply a discount to each product, as needed
     *
     * @return array of cart items
     */
    function update_cart($cart_contents)
    {
        $this->EE->load->helper(array('store'));
 
        // Get the number of products currently in the user's cart
        $count = count($cart_contents['items']);
 
        // Add a $10 discount for 3 or more of the same product
        foreach ($cart_contents['items'] as $item)
        {
            if ($item['item_qty'] >= 3)
            {
                 $cart_contents['order_discount_val'] += 10;
            }
        }
 
        // Add the quantity discount to any existing discount (promo code)
        $cart_contents['order_discount_val'] += $discount;

        // Make sure we don't end up with a negative total
        if ($cart_contents['order_discount_val'] > $cart_contents['order_subtotal_val'])
        {
            $cart_contents['order_discount_val'] = $cart_contents['order_subtotal_val'];
        }

        $cart_contents['order_discount'] = store_format_currency($cart_contents['order_discount_val']);

        // Update the cart total to reflect the new discount
        $cart_contents['order_total_val'] = $cart_contents['order_subtotal_val'] - $cart_contents['order_discount_val'];
        $cart_contents['order_total'] = store_format_currency($cart_contents['order_total_val']);
 
        return $cart_contents;
    }
}
 
/* End of file ext.store_discount.php */
/* Location: /system/expressionengine/third_party/store_discount/ext.store_discount_batch.php */