Add discount to shopping-cart using radio-buttons

I really need someone who can help me :-)

I want to give customers a discount by clicking various radio buttons in my shop.

Visitors can choose max 4 radio buttons. not 5, 6 etc. Is it possible to do this?

My problem now is that the customer can actually get the item for free - or worse - I have to pay to sell goods to them!

Here is the test-code:

    <script src="http://ift.tt/1qRgvOJ"></script>
<input type="checkbox" name="checkbox1" id="checkbox1" class="css-checkbox" value="20" />
<label for="checkbox1" class="css-label">banana</label><br>

<input type="checkbox" name="checkbox2" id="checkbox2" class="css-checkbox" value="20" />
<label for="checkbox2" class="css-label">apple</label><br>

<input type="checkbox" name="checkbox3" id="checkbox3" class="css-checkbox" value="20" />
<label for="checkbox3" class="css-label">biscuit</label><br>

<input type="checkbox" name="checkbox4" id="checkbox4" class="css-checkbox" value="20" />
<label for="checkbox4" class="css-label">jam </label><br>

<input type="checkbox" name="checkbox5" id="checkbox5" class="css-checkbox" value="20" />
<label for="checkbox5" class="css-label">orange </label><br>

<input type="checkbox" name="checkbox6" id="checkbox6" class="css-checkbox" value="20" />
<label for="checkbox6" class="css-label">pinepple </label><br>

<br /><br />

<span id="total"><b>Normal price: </b> 100</span></p>

<script>
var $total = 100;
var $total2 = 100;

$('input:checkbox').on('change', function() {
    if (this.checked)
        $total += -this.value;
    else
        $total -= -this.value;

        if ($total2 > $total)
            $('#total').html('<b>Discount  price: </b>'+$total);
        else
            $('#total').html('<b>Normal price: </b>'+$total);
});
</script>