Dealing with Ajax Data in more then just <div>

[caleman]

So I am trying to build a dynamic order total

subtotal = 12.00
tip 1.00 = 1.00
coupon code 10%off =1.20 <coupon code is 10% off>

total = 11.80

red is input fields

so using the Ajax system I can go lookup the the 10%off via a PHP sql call and it will return that it is 10% off the subtotal.

Coupon: <input name="coupon_code" type="text" onKeyDown="showCoupon(this.value,'txtHintB',this.form.total.value)" size="6" maxlength="6"> <span class="style7"><div id="txtHintB"></div></span><br> 

I get back the discount value and I can pass that back into the page just fine but ... How do I get the total to update based on the discount that was returned?

my ajax code

function showCoupon(str,myloc,ordertotal)
{
    //alert ("str is "+str+" myloc is "+myloc+" the order total is "+ordertotal);
    
    xmlHttp=GetXmlHttpObject();
    //alert("getxmlhttpob is "+xmlHttp);
    if (xmlHttp==null)
      {
      //alert ("Your browser does not support AJAX!");
      return;
      } 
    var url="getcoupon.php";
    url=url+"?coupon_code="+str;
    url=url+"&order_total="+ordertotal;
    url=url+"&sidd="+Math.random();
    xmlHttp.onreadystatechange=function(){stateChangedLoc(myloc)};
    xmlHttp.open("GET",url,true);
    xmlHttp.send(null);
}
function stateChangedLoc(divname)
    { 
    
    if (xmlHttp.readyState==4 || xmlHttp.readyState=="complete")
        { 
        document.getElementById(divname).innerHTML=xmlHttp.responseText;
        }
    } 

and here is my php code for the lookup

if( ($_GET[coupon_code]) != "") { 
//echo " total order is: $_POST[order_total] <br>";
  $sql01 = "SELECT  coupons.coupon_id, coupons.provider_id,  coupons.percent_off,  coupons.dollar_off,  coupons.coupon_type,  coupons.min_purchase,  coupons.coupon_code
                FROM
                  coupons
                WHERE
                  provider_id = 1 AND 
                  coupons.expiration_date > now() AND
                  coupons.coupon_code = '$_GET[coupon_code]'"; 
                
            $output01 = mysql_query($sql01, $db_conn) or die (mysql_error()."<br />Couldn't execute query: $sql01"); 
            $count01 = mysql_num_rows($output01);
            $row01 = mysql_fetch_array($output01);
       if ($count01 == 0) {
            $coupon_error = "The Coupon, $_GET[coupon_code], has expired or does not exist";
            $update = "coupon error";
            } else {
                if ($row01[min_purchase] > (int)$_GET[order_total]) {
                //echo " $row01[min_purchase] is greater then $_POST[order_total] <br>";
                    $update = "coupon error";
                    $coupon_error = "The Order does not meet the minimum amount for this coupon";
                    }
            }
        if  (empty($coupon_error)) {    
            if ($row01[percent_off] != 0) {
                $coupon_discount = (($row01[percent_off] * .01) * $_GET[order_total]);
                $coupon_id = $row01[coupon_id];
                //echo " coupon is: $coupon_discount <br>";
            } else {
                $coupon_discount = $row01[dollar_off] ;
                $coupon_id = $row01[coupon_id];
                //echo " coupon is: $coupon_discount <br>";
            }
         } else {
         echo "$coupon_error";
         }
         echo " " . number_format($coupon_discount,2). " ";
 } // end of $_GET[coupon]