按钮BuyNow不工作超过50美元

Question

嗨我做了一个dinamic按钮，只有当金额<30USD，沙盒打开并且付款完成时，它才能正常工作，IPN也能正常工作。但是，如果我尝试以金额= 50.00或以上的金额付款，则点击按钮会转到www.paypal.com并且付款无法完成，IPN历史记录也不会显示付款。有人能帮我吗。感谢

使用C＃asp.net进行编程

以下是代码：

    protected void ImageButton1_Click(object sender, ImageClickEventArgs e)
    {

        Session["PayPal_Redirected"] = "True";

        StringBuilder url = new StringBuilder();

        // strPP->SandBox
        url.Append(this.strPP + cmd);

        url.Append(HttpUtility.UrlEncode(this.business));
        url.AppendFormat("&item_name={0}", HttpUtility.UrlEncode(this.item_name));
        url.AppendFormat("&image_url={0}", HttpUtility.UrlEncode(this.logo));
        url.AppendFormat("&no_shipping={0}", HttpUtility.UrlEncode(this.no_shipping));
        url.AppendFormat("&return={0}", HttpUtility.UrlEncode(this.return_value));
        url.AppendFormat("&cancel_return={0}", HttpUtility.UrlEncode(this.cancel_return));
        url.AppendFormat("&currency_code={0}", HttpUtility.UrlEncode(this.currency_code));
        url.AppendFormat("&amount={0}", HttpUtility.UrlEncode(this.amount));
        url.AppendFormat("&item_name={0}", HttpUtility.UrlEncode(this.item_name));
        url.AppendFormat("&item_number={0}", HttpUtility.UrlEncode(this.item_number));
        url.AppendFormat("&custom={0}", HttpUtility.UrlEncode(this.os0));
        url.AppendFormat("&src={0}", HttpUtility.UrlEncode(this.src));
        url.AppendFormat("&sra={0}", HttpUtility.UrlEncode(this.sra));
        url.AppendFormat("&notify_url={0}", HttpUtility.UrlEncode(this.notify_url)); 
        Response.Redirect(url.ToString());
        return;
    }