Add $5.00 amount to shopping cart total when checkbox is selected

function AddAmount(checkbox)
{
    if (checkbox.checked)
    {
        var txtCartTotal= document.getElementById("txtCartValue");
        int temp=txtCartTotal.value + 5;
    }
    else
    {
       var txtCartTotal= document.getElementById("txtCartValue");
        int temp=txtCartTotal.value - 5;
    }
}

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<input type="checkbox" name="txtBwayEDUGift" onclick="AddAmount(this)" />

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function AddAmount(checkbox)
{
    var action = "";
    var dGift = 5;
    if (checkbox.checked)
    {
        action = "A";
    }
    else
    {
        action = "S";
    }
    calculateTotal(gift, action)
}

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<input type="checkbox" name="txtBwayEDUGift" onchange="checkboxAdd(this);" checked="checked" />
<script type="text/javascript">
function checkboxAdd( ctl ){
 if(ctl.checked == true ){
   calculateTotal(5, "A");
  } else {
   calculateTotal( 5, "S");
  }
} 

It appears the functionality is there and is working to an extent. But instead of adding the $5.00 charge to the order, it is instead keeping the orginal amount and only reducing the amount when unchecked. It is not adding the $5.00 to the order amount when checked. Please see my screenshots attached. With the $5.00 added the order amount should be $1,126.00 in my example. I hope this makes sense. Please help in anyway possible. thank you.


</script>  

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