定义折扣sales.xml

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Sales:etc/sales.xsd">
    <section name="quote">
        <group name="totals">
            <item name="merge_shipping_discount" instance="Aiweline\CategoryProductOrderMerger\Model\Quote\MergeDiscount" sort_order="500"/>
        </group>
    </section>
</config>

代码：MergeDiscount.php

<?php
/**
 * 文件信息
 * 作者：邹万才
 * 网名：秋风雁飞(可以百度看看)
 * 网站：www.aiweline.com/bbs.aiweline.com
 * 工具：PhpStorm
 * 日期：2020/10/17
 * 时间：14:41
 * 描述：此文件源码由Aiweline（秋枫雁飞）开发，请勿随意修改源码！
 */

namespace Aiweline\CategoryProductOrderMerger\Model\Quote;


use Aiweline\CategoryProductOrderMerger\Helper\Data;
use Aiweline\CategoryProductOrderMerger\Model\CheckManagement;

class MergeDiscount extends \Magento\Quote\Model\Quote\Address\Total\AbstractTotal
{
    /**
     * Discount calculation object
     *
     * @var \Magento\SalesRule\Model\Validator
     */
    protected $calculator;

    /**
     * Core event manager proxy
     *
     * @var \Magento\Framework\Event\ManagerInterface
     */
    protected $eventManager = null;

    /**
     * @var \Magento\Store\Model\StoreManagerInterface
     */
    protected $storeManager;

    /**
     * @var \Magento\Framework\Pricing\PriceCurrencyInterface
     */
    protected $priceCurrency;
    /**
     * @var Data
     */
    private $data;
    /**
     * @var \Magento\Directory\Model\CurrencyFactory
     */
    private $currencyFactory;

    /**
     * @param Data $data
     * @param \Magento\Framework\Event\ManagerInterface $eventManager
     * @param \Magento\Store\Model\StoreManagerInterface $storeManager
     * @param \Magento\SalesRule\Model\Validator $validator
     * @param \Magento\Framework\Pricing\PriceCurrencyInterface $priceCurrency
     */
    public function __construct(
        Data $data,
        \Magento\Framework\Event\ManagerInterface $eventManager,
        \Magento\Store\Model\StoreManagerInterface $storeManager,
        \Magento\SalesRule\Model\Validator $validator,
        \Magento\Framework\Pricing\PriceCurrencyInterface $priceCurrency,
        \Magento\Directory\Model\CurrencyFactory $currencyFactory
    )
    {
        $this->setCode('merge_shipping_discount');
        $this->eventManager = $eventManager;
        $this->calculator = $validator;
        $this->storeManager = $storeManager;
        $this->priceCurrency = $priceCurrency;
        $this->data = $data;
        $this->currencyFactory = $currencyFactory;
    }

    /**
     * Collect address discount amount
     *
     * @param \Magento\Quote\Model\Quote $quote
     * @param \Magento\Quote\Api\Data\ShippingAssignmentInterface $shippingAssignment
     * @param \Magento\Quote\Model\Quote\Address\Total $total
     * @return $this
     * @SuppressWarnings(PHPMD.CyclomaticComplexity)
     * @throws \Magento\Framework\Exception\NoSuchEntityException
     */
    public function collect(
        \Magento\Quote\Model\Quote $quote,
        \Magento\Quote\Api\Data\ShippingAssignmentInterface $shippingAssignment,
        \Magento\Quote\Model\Quote\Address\Total $total
    )
    {
        parent::collect($quote, $shippingAssignment, $total);
        if (!$quote->getCustomerId()) return $this;
        $discountAmount = -floatval($quote->getData('check_merge') ? $quote->getShippingAddress()->getShippingAmount() : 0);
        $baseDiscountAmount = $this->convertPrice($discountAmount);
        $quote->getShippingAddress()->setShippingDiscountAmount($discountAmount);
        $quote->setData('merge_origin_shipping_amount', $discountAmount);
        if ($this->data->isEnabled() && $discountAmount) {
            $label = $this->data->getMergeDiscountLabel();
            if ($total->getDiscountDescription()) {
                // If a discount exists in cart and another discount is applied, the add both discounts.
                $appliedCartDiscount = $total->getDiscountAmount();
                $discountAmount = $total->getDiscountAmount() + $discountAmount;
                $label = $total->getDiscountDescription() . ', ' . $label;
            }

            $total->setDiscountDescription($label);
            $total->setDiscountAmount($discountAmount);
            $total->setBaseDiscountAmount($baseDiscountAmount);
            $total->setSubtotalWithDiscount($total->getSubtotalWithDiscount() + $discountAmount);
            $total->setBaseSubtotalWithDiscount($total->getBaseSubtotal() + $baseDiscountAmount);
            if (isset($appliedCartDiscount)) {
                $total->addTotalAmount($this->getCode(), $discountAmount - $appliedCartDiscount);
                $total->addBaseTotalAmount($this->getCode(), $baseDiscountAmount - $appliedCartDiscount);
            } else {
                $total->addTotalAmount($this->getCode(), $discountAmount);
                $total->addBaseTotalAmount($this->getCode(), $baseDiscountAmount);
            }
        }

        return $this;
    }

    /**
     * 货币价格转化
     *
     * @param $amountValue
     * @param null $current_currency_code
     * @param null $base_currency
     * @return float
     * @throws \Magento\Framework\Exception\NoSuchEntityException
     */
    public function convertPrice($amountValue, $current_currency_code = null, $base_currency = null)
    {
        $current_currency_code = $current_currency_code ?? $currentCurrency = $this->storeManager->getStore()->getCurrentCurrency()->getCode();
        $base_currency = $base_currency ?? $baseCurrency = $this->storeManager->getStore()->getBaseCurrency()->getCode();
        if ($current_currency_code != $base_currency) {
            $rate = $this->currencyFactory->create()->load($current_currency_code)->getAnyRate($base_currency);
            $amountValue = $amountValue * $rate;
        }

        return $amountValue;
    }

    /**
     * Add discount total information to address
     *
     * @param \Magento\Quote\Model\Quote $quote
     * @param \Magento\Quote\Model\Quote\Address\Total $total
     * @return array|null
     */
    public function fetch(\Magento\Quote\Model\Quote $quote, \Magento\Quote\Model\Quote\Address\Total $total)
    {
        $result = null;
        if ($this->data->isEnabled() && $amount = $quote->getData('merge_origin_shipping_amount')) {
            // ONLY return 1 discount. Need to append existing
            //see app/code/Magento/Quote/Model/Quote/Address.php

            $description = $total->getDiscountDescription();
            $result = [
                'code' => $this->getCode(),
                'title' => strlen($description) ? __('Discount (%1)', $description) : __('Discount'),
                'value' => $amount
            ];
        }
        return $result;
    }
}