1.重写模型Model 可以直接采用同名模块覆盖的方法，例如下面。

要重新Discount.php 文件，直接在\app\code\local\Mage\SalesRule\Model\Quote\ 下面建立
Discount.php

D:\www\lollicupStore2\app\code\local\Mage\SalesRule\Model\Quote\Discount.php

如下代码：

<?php
/**
 * Magento
 *
 * NOTICE OF LICENSE
 *
 * This source file is subject to the Open Software License (OSL 3.0)
 * that is bundled with this package in the file LICENSE.txt.
 * It is also available through the world-wide-web at this URL:
 * http://opensource.org/licenses/osl-3.0.php
 * If you did not receive a copy of the license and are unable to
 * obtain it through the world-wide-web, please send an email
 * to license@magento.com so we can send you a copy immediately.
 *
 * DISCLAIMER
 *
 * Do not edit or add to this file if you wish to upgrade Magento to newer
 * versions in the future. If you wish to customize Magento for your
 * needs please refer to http://www.magento.com for more information.
 *
 * @category    Mage
 * @package     Mage_SalesRule
 * @copyright  Copyright (c) 2006-2018 Magento, Inc. (http://www.magento.com)
 * @license    http://opensource.org/licenses/osl-3.0.php  Open Software License (OSL 3.0)
 */

/**
 * Discount calculation model
 *
 * @category    Mage
 * @package     Mage_SalesRule
 * @author      Magento Core Team <core@magentocommerce.com>
 */
class Mage_SalesRule_Model_Quote_Discount extends Mage_Sales_Model_Quote_Address_Total_Abstract
{
    /**
     * Discount calculation object
     *
     * @var Mage_SalesRule_Model_Validator
     */
    protected $_calculator;

    /**
     * Initialize discount collector
     */
    public function __construct()
    {
        $this->setCode('discount');
        $this->_calculator = Mage::getSingleton('salesrule/validator');
    }

    /**
     * Collect address discount amount
     *
     * @param   Mage_Sales_Model_Quote_Address $address
     * @return  Mage_SalesRule_Model_Quote_Discount
     */
    public function collect(Mage_Sales_Model_Quote_Address $address)
    {
        //print_r($address);

        Mage_Sales_Model_Quote_Address_Total_Abstract::collect($address);
        $quote = $address->getQuote();
        $store = Mage::app()->getStore($quote->getStoreId());
        $this->_calculator->reset($address);

        $items = $this->_getAddressItems($address);
        if (!count($items)) {
            return $this;
        }

        $couponCode = $quote->getCouponCode();
        $couponArray = explode(',', $couponCode);

        //exit('coupon price');
        foreach ($couponArray as $couponCode) {
            $this->_calculator->init($store->getWebsiteId(), $quote->getCustomerGroupId(), $couponCode);
            $this->_calculator->initTotals($items, $address);

            $eventArgs = array(
                'website_id' => $store->getWebsiteId(),
                'customer_group_id' => $quote->getCustomerGroupId(),
                'coupon_code' => $couponCode,
            );

            $address->setDiscountDescription(array());
            $items = $this->_calculator->sortItemsByPriority($items);
            foreach ($items as $item) {
                if ($item->getNoDiscount()) {
                    $item->setDiscountAmount(0);
                    $item->setBaseDiscountAmount(0);
                } else {
                    /**
                     * Child item discount we calculate for parent
                     */
                    if ($item->getParentItemId()) {
                        continue;
                    }

                    $eventArgs['item'] = $item;
                    Mage::dispatchEvent('sales_quote_address_discount_item', $eventArgs);

                    if ($item->getHasChildren() && $item->isChildrenCalculated()) {
                        foreach ($item->getChildren() as $child) {
                            $this->_calculator->process($child);
                            $eventArgs['item'] = $child;
                            Mage::dispatchEvent('sales_quote_address_discount_item', $eventArgs);

                            $this->_aggregateItemDiscount($child);
                        }
                    } else {
                        $this->_calculator->process($item);
                        $this->_aggregateItemDiscount($item);
                    }
                }
            }

            /**
             * process weee amount
             */
            if (Mage::helper('weee')->isEnabled() && Mage::helper('weee')->isDiscounted($store)) {
                $this->_calculator->processWeeeAmount($address, $items);
            }

            /**
             * Process shipping amount discount
             */
            $address->setShippingDiscountAmount(0);
            $address->setBaseShippingDiscountAmount(0);
            if ($address->getShippingAmount()) {
                $this->_calculator->processShippingAmount($address);
                $this->_addAmount(-$address->getShippingDiscountAmount());
                $this->_addBaseAmount(-$address->getBaseShippingDiscountAmount());
            }

            $this->_calculator->prepareDescription($address);
        }

        return $this;
    }

    /**
     * Aggregate item discount information to address data and related properties
     *
     * @param   Mage_Sales_Model_Quote_Item_Abstract $item
     * @return  Mage_SalesRule_Model_Quote_Discount
     */
    protected function _aggregateItemDiscount($item)
    {
        $this->_addAmount(-$item->getDiscountAmount());
        $this->_addBaseAmount(-$item->getBaseDiscountAmount());
        return $this;
    }

    /**
     * Add discount total information to address
     *
     * @param   Mage_Sales_Model_Quote_Address $address
     * @return  Mage_SalesRule_Model_Quote_Discount
     */
    public function fetch(Mage_Sales_Model_Quote_Address $address)
    {
        $amount = $address->getDiscountAmount();

        if ($amount != 0) {
            $description = $address->getDiscountDescription();
            if (strlen($description)) {
                $title = Mage::helper('sales')->__('Discount (%s)', $description);
            } else {
                $title = Mage::helper('sales')->__('Discount');
            }
            $address->addTotal(array(
                'code'  => $this->getCode(),
                'title' => $title,
                'value' => $amount
            ));
        }
        return $this;
    }
}