Add up Checkbox Values and Write to Input Field

I have a form with some checkboxes and a Javascript snippet where the checkbox values get added up and written to a <span>. That works fine but I would really like it to write to an input text field instead of the <span>.

Here is the checkbox section of my form:

<section id="extra-features">
<div class="checkbox">
<label><input type="checkbox" name="checkbox" id="outside" class="sum" value="10" data-toggle="checkbox"> Outside Wash</label>
</div><br/>
<div class="checkbox">
<label><input type="checkbox" name="checkbox" id="aclean" class="sum" value="39" data-toggle="checkbox"> A - Clean: Wash Vacuum, Windows, Wheels/Tires, Wax</label>
</div><br/>
<div class="checkbox">
<label><input type="checkbox" name="checkbox" id="bclean" class="sum" value="0" data-toggle="checkbox"> B - Clean: Same as A above <em>PLUS:</em> Shampoo Interior, Clean/Dress Interior Panels, Remove Bugs/Tar.</label>
</div><br/>
<div class="checkbox">
<label><input type="checkbox" name="checkbox" id="cclean" class="sum" value="109" data-toggle="checkbox"> C - Clean: Same as B above <em>PLUS:</em> Compound Polish Exterior, Clean/Dress Moldings as Needed.    </label>
</div>
</section>

This is the <span> that the javascript is writing to currently:

<span id="payment-total" style="text-decoration:underline;">0</span>

And here is the javascript:

window.onload=function(){
var inputs = document.getElementsByClassName('sum'),
    total  = document.getElementById('payment-total');

 for (var i=0; i < inputs.length; i++) {
    inputs[i].onchange = function() {
        var add = this.value * (this.checked ? 1 : -1);
        total.innerHTML = parseFloat(total.innerHTML) + add
    }
  }
}