Webmoney merchant form

On page:
<asp:Content ID="content" ContentPlaceHolderID="afterform" runat="server">
<uc:WebMoneyForm id="wmpay" runat="server" />
</asp:Content>
In code:
wmpay.Amount = 1.5;
wmpay.Description = "некий товар";
wmpay.OrderID = 1001;
wmpay.Purse = "Uxxxxxxxxxxxx";
wmpay.TestMode = true;

public class WebMoneyForm : System.Web.UI.Control
{
 public string Action { get; set; }
 public string Method { get; set; }
 public double Amount { get; set; }
 public string Description { get; set; }
 public bool TestMode { get; set; }
 public int OrderID { get; set; }
 public string Purse { get; set; }

 public Dictionary Params { get; private set; }

 public WebMoneyForm()
 {
  Method = "post";
  Action = "https://merchant.webmoney.ru/lmi/payment.asp";

  Params = new Dictionary<string, string>();
 }

 public void AppParam(string name, string value)
 {
  if(Params.ContainsKey(name))
  {
   Params[name] = value;
  }
  else
  {
   Params.Add(name, value);
  }
 }

 protected override void Render(System.Web.UI.HtmlTextWriter writer)
 {
  writer.AddAttribute("id", ID);
  writer.AddAttribute("name", ID);
  writer.AddAttribute("action", Action);
  writer.AddAttribute("method", Method);
  writer.RenderBeginTag(HtmlTextWriterTag.Form);

  AppParam("LMI_PAYMENT_AMOUNT", Math.Round(Amount, 2).ToString());
  AppParam("LMI_PAYMENT_DESC_BASE64", Convert.ToBase64String(System.Text.Encoding.UTF8.GetBytes(Description)));

  AppParam("LMI_PAYMENT_NO", OrderID.ToString());
  AppParam("LMI_PAYEE_PURSE", Purse);

  if(TestMode) AppParam("LMI_SIM_MODE", "0");

  foreach(var item in Params)
  {
   AddHidden(writer, item.Key, item.Value);
  }

  writer.RenderEndTag();
 }

 private void AddHidden(System.Web.UI.HtmlTextWriter writer, string name, string value)
 {
  writer.Write(string.Format("<input type='hidden' name='{0}' value='{1}' />\n", name, value));
 }
}