How do you add radio button valus together, yet differentiate them if they have the same value?

So I'm working on an order form, where people can select different quantities of items, and it then uses JS to calculate to the totals. So for example you can select 2 Brown hats ($15 each) and the TOTAL automatically becomes $30. In the POST page I can then pull the name of the text box and know they ordered 2 brown hats.

Here is my problem: I want to set up a radio button option that will allow them to select one item only, and have it add to that final total here is what it looks like:

 type="radio" value="0" name="Syllabus" checked="No" /> Paper ($0) />

 type="radio" value="0" name="Syllabus" checked="No" /> USB ($0) />

 type="radio" value="20" name="Syllabus" checked="No" /> Both ($20) />

So how can I use JS to add $20 to the total if "BOTH" is selected? And then of course $20 is removed if they select one of the other options. And finally, if they do select PAPER or USB, how do I know in the final output what they selected, since the value will be 0 either way?

Thanks for the help!

------ IF IT HELPS ANYONE HERE IS THE CODE I USE TO ADD THE VALUES OF THE OTHER TEXT BOXES, I THEN PAD THE TOTAL WITH ZEROS AND ROUND IT ------

// JavaScript Document

    function CalculateTotal(frm) {
    var order_total = 0

    var syllabuscost;

    var radios = document.getElementsByTagName('input');
    var radiovalue;
    for (var z = 0; z < radios.length; z++) {
        if (radios[z].type === 'radio' && radios[z].checked) {
            // get value, set checked flag or do whatever you need to
            radiovalue = radios[z].value; 

            //Change the syllabus cost depending on the value of the radio button
            if (radiovalue == "Paper" || radiovalue == "USB") {
                syllabuscost = 0
            }
            else if (radiovalue == "Both") {
                syllabuscost = 20
            }      
        }
    }

    // Run through all the form fields
    for (var i=0; i < frm.elements.length; ++i) {

        // Get the current field
        form_field = frm.elements[i]

        // Get the field's name
        form_name = form_field.name

        // Is it a "product" field?
        if (form_name.substring(0,4) == "PROD") {

            // If so, extract the price from the name
            item_price = parseFloat(form_name
                                                

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