Valor decimal depois da virgula não calcula

[miro]

Estou usando essa função, mais não calcula os centavos exemplo: 24,50 + 12,00 = 36,00

E os valores exemplo 1.230,00 fica assim 1230.00

O que pode estar ocorrendo para não calculo dos centavos, e regra para deixar ponto e virgula.

AONDE PARECE O VALOR

<?php echo SUB_TITLE_SUB_TOTAL; ?></b><span id="total_display"><?php echo $currencies->format($cart->show_total()); ?></span><input type="hidden" id="sub_total" value="<?php echo $cart->total; ?>" />

FUNÇÃO

<script type="text/javascript">
var xmlhttp;
function loadXMLDoc(url)
{
var postcode = document.getElementById('postcode').value;
if (window.XMLHttpRequest)
  {// code for IE7+, Firefox, Chrome, Opera, Safari
  xmlhttp=new XMLHttpRequest();
  xmlhttp.onreadystatechange=stateChange;
  xmlhttp.open("GET",url+"&postcode="+postcode,true);
  xmlhttp.send(null);
  }
else
  {// code for IE6, IE5
  xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
  xmlhttp.onreadystatechange=stateChange;
  xmlhttp.open("GET",url+"&postcode="+postcode,true);
  xmlhttp.send();
  }
}
var xmlhttp;
function enviaReq(url, tipo)
{
var postcode = document.getElementById('postcode').value;
var shipping = document.getElementById('shipping_'+tipo).value;
var valor_frete = document.getElementById('valor_'+tipo).value;
var sub_total = document.getElementById('sub_total').value;
var total = (parseFloat(valor_frete) + parseFloat(sub_total)).toFixed(2);
document.getElementById("total_display").innerHTML= "+total;

if (window.XMLHttpRequest)
  {// code for IE7+, Firefox, Chrome, Opera, Safari
  xmlhttp=new XMLHttpRequest();
  xmlhttp.onreadystatechange=stateChange;
  xmlhttp.open("GET",url+"&postcode="+postcode+"&shipping="+shipping,true);
  xmlhttp.send(null);
  }
else
  {// code for IE6, IE5
  xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
  xmlhttp.onreadystatechange=stateChange;
  xmlhttp.open("GET",url+"&postcode="+postcode+"&shipping="+shipping,true);
  xmlhttp.send();
  }
}
function stateChange()
{
if (xmlhttp.readyState==4)
  {
  if (xmlhttp.status==200)
    {
    // process whatever has been sent back here
    document.getElementById('test').innerHTML=xmlhttp.responseText;
    }
  }
} 
</script>