要free shipping在结帐页面上自动选择，您可以覆盖Magento_Checkout模块中的javascript 。您可以通过两种方式实现

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1]通过创建模块，

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Namespace/Module/registration.php

<?php\Magento\Framework\Component\ComponentRegistrar::register(
    \Magento\Framework\Component\ComponentRegistrar::MODULE,
    'Namespace_Module',    __DIR__);

Namespace/Module/etc/module.xml

<?xml version="1.0"?><config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
    <module name="Namespace_Module" setup_version="1.0.0">
        <sequence>
            <module name="Magento_Checkout"/>
        </sequence>
    </module></config>

然后创建requirejs来覆盖checkout-data-resolverjs，

为了这

Namespace/Module/view/frontend/requirejs-config.js

var config = {    map: {        '*': {            'Magento_Checkout/js/model/checkout-data-resolver': 'Namespace_Module/js/model/checkout-data-resolver'
        }
    }};

现在使用相同的路径checkout-data-resolver.js从Magento_Checkout/view/frontend/web/js/model我们的模块中复制Namespace_Module/view/frontend/web/js/model

之后，在函数内部更改条件 resolveShippingRates

从：

............................
     if (ratesData.length == 1) {                    //set shipping rate if we have only one available shipping rate
                    selectShippingMethodAction(ratesData[0]);
                    return;
                }.............................

至

.................................
     var selectedShippingRate = checkoutData . getSelectedShippingRate();        var availableRate = false;        if (selectedShippingRate == null) { // custom code start here
            for (var i = 0;i < ratesData . length;i++){                if (ratesData[i] . method_code == 'freeshipping') {
                    selectShippingMethodAction(ratesData[i]);
                    return;
                }
            }
            } else { // custom code end here
            if (ratesData . length == 1) {                //set shipping rate if we have only one available shipping rate
                selectShippingMethodAction(ratesData[0]);                return;
            }
        }.....................................

更换Namespace/Module/你的YOUR_NAMESPACE和YOUR_MODULENAME

1. 激活模块： php bin/magento module:enable Namespace_Module
2. 运行安装程序升级： php bin/magento setup:upgrade
3. 进行静态部署： php bin/magento setup:static-content:deploy

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2]通过使用主题

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只需复制checkout-data-resolver.js到您的自定义主题中，但请确保它的路径应该是，

app\design\yourvendor\yourthemename\Magento_Checkout\view\frontend\web\js\model

复制checkout-data-resolver.js具有resolveShippingRates功能的更新后，

  resolveShippingRates: function (ratesData) {               var selectedShippingRate = checkoutData . getSelectedShippingRate();               var availableRate = false;
               if (selectedShippingRate == null) { // custom code start here
                    for (var i = 0;i < ratesData . length;i++){                            if (ratesData[i] . method_code == 'freeshipping') {
                                selectShippingMethodAction(ratesData[i]);
                                return;
                            }
                        }
                    } else { // custom code end here
                            if (ratesData . length == 1) {                                    //set shipping rate if we have only one available shipping rate
                                    selectShippingMethodAction(ratesData[0]);                                    return;
                            }
                    }
                if (quote.shippingMethod()) {
                    availableRate = _.find(ratesData, function (rate) {                        return rate.carrier_code == quote.shippingMethod().carrier_code &&
                            rate.method_code == quote.shippingMethod().method_code;
                    });
                }
                if (!availableRate && selectedShippingRate) {
                    availableRate = _.find(ratesData, function (rate) {                        return rate.carrier_code + '_' + rate.method_code === selectedShippingRate;
                    });
                }
                if (!availableRate && window.checkoutConfig.selectedShippingMethod) {
                    availableRate = true;
                    selectShippingMethodAction(window.checkoutConfig.selectedShippingMethod);
                }
                //Unset selected shipping method if not available
                if (!availableRate) {
                    selectShippingMethodAction(null);
                } else {
                    selectShippingMethodAction(availableRate);
                }
            },

将js文件覆盖为您的自定义主题后，执行以下命令，

1. php bin / magento缓存：刷新
2. php bin / magento设置：静态内容：部署