Google Checkout回调问题

我正在尝试（使用沙箱帐户）使用谷歌结账来销售商品。我正在向用户显示一个表单，该表单会导致立即购买按钮

<form method="POST" action="https://sandbox.google.com/checkout/api/checkout/v2/checkoutForm/Merchant/..." accept-charset="utf-8">  
    <input type="hidden" name="item_name_1" value="Test"/>  
    <input type="hidden" name="item_description_1" value="An item "/>  
    <input type="hidden" name="item_quantity_1" value="1"/>  
    <input type="hidden" name="item_price_1" value="1.50"/> 
    <input type="hidden" name="item_currency_1" value="GBP"/>    
    <input type="hidden" name="_charset_"/>   
    <input type="hidden" name="checkout-flow-support.merchant-checkout-flow-support.continue-shopping-url" value="redirect to this url"/>
    <input type="image" name="Google Checkout" alt="Fast checkout through Google" src="http://sandbox.google.com/checkout/buttons/checkout.gif?merchant_id=....&w=180&h=46&style=white&variant=text&loc=en_US" height="46" width="180"/>  
</form>

在谷歌卖家账户上，我也设置了回拨的网址。在这个例子中，我使用的是谷歌文档示例中的php文件

// Include Google Checkout PHP Client Library
include ("GlobalAPIFunctions.php");
// Include Response Message Processor
include ("ResponseHandlerAPIFunctions.php");
// Retrieve the XML sent in the HTTP POST request to the ResponseHandler
$xml_response = $HTTP_RAW_POST_DATA;
// Get rid of PHP's magical escaping of quotes 
if (get_magic_quotes_gpc()) {
    $xml_response = stripslashes($xml_response);
}
// Log the XML received in the HTTP POST request
LogMessage($GLOBALS["logfile"], $xml_response);
/*
 * Call the ProcessXmlData function, which is defined in
 * ResponseHandlerAPIFunctions.php. The ProcessXmlData will route 
 * the XML data to the function that handles the particular type
 * of XML message contained in the POST request.
 */
ProcessXmlData($xml_response);

问题是，一旦我买了一件商品，我就不会接到任何电话。没有错误消息，什么都没有，所以我怎么能看到发生了什么？希望有人能帮我解决

感谢