เรื่องการบวกกัน ของ xml น่ะครับ แบบว่า บวกมันไม่ได้ ต้องกำหนดอย่างไรดีครับ

function allamount(){
  numitems=document.getElementById('tbl_sitems').rows.length-1;
  itemsqty = document.getElementsByName('itemsqty[]');
  itemstax = document.getElementsByName('itemstax[]');
  itemstotaltax = document.getElementsByName('itemstotaltax[]');
  itemsuprice = document.getElementsByName('itemsuprice[]');
  itemsprice = document.getElementsByName('itemsprice[]');
  itemsprice2 = document.getElementsByName('itemsprice2[]');
  itemstotalprice = document.getElementsByName('itemstotalprice[]');
  var tax = 0;
  var total = 0;
  for(i=0;i<numitems;i++){
    iqty = parseInt(itemsqty[i].value);	 	//Items quantity
    tip = iqty*toCurrency(parseFloat(itemsprice2[i].value));  //Calculate total item price
    titax = iqty*toCurrency(parseFloat(itemsuprice[i].value))*toCurrency(parseFloat(itemstax[i].value)/100); 	//Calculate total item tax
    tax += titax;
    total += tip;
    itemstotaltax[i].value = toCurrency(titax);
    itemstotalprice[i].value = toCurrency(tip);
  }
  total = toCurrency(total);
  total2 = toCurrency(total*7/100);
  total3 = total+total2;
  tax = toCurrency(tax);
  var subtotal = toCurrency(total-tax);
  document.getElementById('vtax').value=tax;
  document.getElementById('vattt').value=total2;
  document.getElementById('vtotal').value=total3;
  document.getElementById('vtotal2').focus();
}