<!--免邮-->
<event name="sales_quote_collect_totals_before">
    <observer name="Aiweline_Upsell::sales_quote_collect_totals_before" instance="Aiweline\Upsell\Observer\Quote\SalesQuoteCollectTotalsBefore" />
</event>

代码：

<?php
/**
 * @Author       秋枫雁飞
 * @Email        aiweline@qq.com/1714255949@qq.com
 * @Desc         文件由Aiweline(秋枫雁飞)编写，若有升级需要
 *               建议不要随意修改文件源码。
 **/

namespace Aiweline\Upsell\Observer\Quote;


use Magento\Framework\Event\Observer;

class SalesQuoteCollectTotalsBefore implements \Magento\Framework\Event\ObserverInterface
{

    /**
     * @inheritDoc
     */
    public function execute(Observer $observer)
    {
        $quote = $observer->getEvent()->getData('quote');
        if (!$quote) return;
        $shippingAddress = $quote->getShippingAddress()->setFreeShipping(1);
        $shippingAddress->setCollectShippingRates(true)
            ->collectShippingRates()
            ->setShippingMethod('freeshipping_freeshipping');
    }
}