In order to select a radio button when clicking the label, there is no need for JQuery code. You can achieve this by using the following code:
<label for="regular">
<input type='radio' name='product-choice' value='regular' id="regular" /> regular
</label>
To make this work, ensure that the ID attribute is set in the textbox and use that ID attribute value in the label's 'for' attribute.
Below is the complete code example:
<form id='form-product-choices' action=''>
<main role="main">
<section id="p1">
<ul>
<li>
<label for="regular">
<input type='radio' name='product-choice' value='regular' id="regular" /> regular
</label>
</li>
<li>
<label for="double">
<input type='radio' name='product-choice' value='double' id="double" /> double
</label>
</li>
<li>
<label for="xl">
<input type='radio' name='product-choice' value='max-xl' id="xl" /> xl
</label>
</li>
</ul>
<a class="quizbut">Next</a>
</section>
<section id="p4">
<a id='action-link' class='quizbut' href='javascript:void(0)'>Click to continue</a>
</section>
</main>
</form>
<script type="text/javascript">
$("#action-link").click(function(){
var getVal = $('input[name=product-choice]:checked').val();
window.location.href='https://example.com/test.cfm?product-choice='+getVal;
});
</script>