PayPal 如果数据库更新失败，则回滚/退款 IPN

我想知道如何处理客户被收取数字商品费用的情况，然后调用IPN，我交付数字商品。如果在交付过程中出现问题会怎样？这个案子应该如何处理？在这种情况下，我可以做些什么来取消/退款？

我的 ipn 代码基于我找到的示例

protected void Page_Load(object sender, EventArgs e)
{
    //Post back to either sandbox or live
    string strSandbox = "https://www.sandbox.paypal.com/cgi-bin/webscr";
    // string strLive = "https://www.paypal.com/cgi-bin/webscr";
    HttpWebRequest req = (HttpWebRequest)WebRequest.Create(strSandbox);
    //Set values for the request back
    req.Method = "POST";
    req.ContentType = "application/x-www-form-urlencoded";
    byte[] param = Request.BinaryRead(HttpContext.Current.Request.ContentLength);
    string strRequest = Encoding.ASCII.GetString(param);
    strRequest += "&cmd=_notify-validate";
    req.ContentLength = strRequest.Length;
    //for proxy
    //WebProxy proxy = new WebProxy(new Uri("http://url:port#"));
    //req.Proxy = proxy;
    //Send the request to PayPal and get the response
    StreamWriter streamOut = new StreamWriter(req.GetRequestStream(), System.Text.Encoding.ASCII);
    streamOut.Write(strRequest);
    streamOut.Close();
    StreamReader streamIn = new StreamReader(req.GetResponse().GetResponseStream());
    string strResponse = streamIn.ReadToEnd();
    streamIn.Close();
    if (strResponse == "VERIFIED")
    {
        //UPDATE YOUR DATABASE
        //check the payment_status is Completed
        //check that txn_id has not been previously processed
        //check that receiver_email is your Primary PayPal email
        //check that payment_amount/payment_currency are correct
        //process payment
    }
    else if (strResponse == "INVALID")
    {
        //UPDATE YOUR DATABASE
    }
    else
    {  //UPDATE YOUR DATABASE
    }
}

如果我的数据库更新失败怎么办？