function getXMLHttp()
{
  var xmlHttp

  try
  {
    //Firefox, Opera 8.0+, Safari
    xmlHttp = new XMLHttpRequest();
  }
  catch(e)
  {
    //Internet Explorer
    try
    {
      xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
    }
    catch(e)
    {
      try
      {
        xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
      }
      catch(e)
      {
        alert("Your browser does not support AJAX!")
        return false;
      }
    }
  }
  return xmlHttp;
}
var http = getXMLHttp();

function getPostage(){
    var selectmenu = document.getElementById('country');
 	var keyvalue = selectmenu.options[selectmenu.selectedIndex].value;
	http.open('get', 'shipping_estimator_rpc.php?country=' 
			+ keyvalue
			+ '&zip='
			+ document.getElementById('zip').value);
	/* Define a function to call once a response has been received. This will be our
		handleProductCategories function that we define below. */
	http.onreadystatechange = handlePostage; 
	/* Send the data. We use something other than null when we are sending using the POST
		method. */
	http.send(null);
}

/* Function called to handle the list that was returned from the internal_request.php file.. */
function handlePostage(){
	/* Make sure that the transaction has finished. The XMLHttpRequest object 
		has a property called readyState with several states:
		0: Uninitialized
		1: Loading
		2: Loaded
		3: Interactive
		4: Finished */
	if(http.readyState == 4){ //Finished loading the response
		/* We have got the response from the server-side script,
			let's see just what it was. using the responseText property of 
			the XMLHttpRequest object. */
		var response = http.responseText;
		/* And now we want to change the product_categories <div> content.
			we do this using an ability to get/change the content of a page element 
			that we can find: innerHTML. */
		document.getElementById('postage_amount').innerHTML = response;
	}
}