function addCommas(nStr)
{
	  nStr += '';
	  x = nStr.split('.');
	  x1 = x[0];
	  x2 = x.length > 1 ? '.' + x[1] : '';
	  var rgx = /(\d+)(\d{3})/;
	  while (rgx.test(x1)) {
		    x1 = x1.replace(rgx, '$1' + ',' + '$2');
	  }
	  return x1 + x2;
}
 
function calculate_total (f)
{
    var total = 0;
    
    var price_field = 'product_variation_'+f.product_variation_id.value+'_price';
    
    total = f.elements[price_field].value*f.elements['num'].value;
    
    document.getElementById('_price').innerHTML = addCommas(total);
}