Problem in executing IPN script of paypal

i am using PPFrame.html as the paypal page
<form method="post" action= "https://www.sandbox.paypal.com/cgi-bin/webscr" target="_top">
                <input type="hidden" name="cmd" value="_xclick" />
                <input type="hidden" name="business" value="pay@mysite.com" />
                <input type="hidden" name="item_name" value="itemname" />
                <input type="hidden" name="item_number" value="1" />
                <input type="hidden" name="amount" value="0.10" />
                <input type="hidden" name="no_shipping" value="1" />
                <input type="hidden" name="custom" value="3" />                
                <input type="hidden" name="return" value="http://www.mysite.com/Success.aspx" />
                <input type="submit" value="Buy This"  align="middle" />
            </form>

my IPNProcess.aspx holds the DataBase manipulation code like altering a feild date_of_payment in 'list' table.
protected void Page_Load(object sender, EventArgs e)
    {
          string id = Request.Form["custom"];
         // here i am using the SQL to alter my table
       
        PaymentStatus = HttpContext.Current.Request.Form["payment_status"];
        PaymentType = HttpContext.Current.Request.Form["payment_type"];
        PendingReason = HttpContext.Current.Request.Form["pending_reason"];
        TXN_ID = HttpContext.Current.Request.Form["txn_id"];
        TXN_Type = HttpContext.Current.Request.Form["txn_type"];
        PaymentFee = HttpContext.Current.Request.Form["mc_fee"];
        PaymentGross = HttpContext.Current.Request.Form["mc_gross"];
        NotifyVersion = HttpContext.Current.Request.Form["notify_version"];
        VerifySign = HttpContext.Current.Request.Form["verify_sign"];
//sending mail to buyer
    }

my Success.aspx page contains only a link to back to HomePage.

i dont know whether i am right or wrong?. or do i need to do some settings in my sandbox paypal account?