JSFiddle - React, Tailwind, and code Playground

HTML

<div id="checkout-shipping-method-load">
    <select id="shipping-method">
    <option value="option1">1 twister ($14.99 + $6.99 S&H)</option>
    <option value="option2">1 twister ($14.99 + $6.99 S&H)</option>
    </select>
    </div>
    
    <input type="radio" class="radio" checked="checked" id="s_method_flatrate2_flatrate2" value="flatrate2_flatrate2" name="shipping_method">
 <input type="radio" class="radio" id="s_method_flatrate_flatrate" value="flatrate_flatrate" name="shipping_method">

JavaScript

$('#shipping-method').change(function(){
 var optionSelected = $("option:selected", this);
 $(".radio:eq("+ optionSelected.index() +")").prop("checked", true);
});