I m going to answer my own question here just in case this gets referred to by someone else. Basically, I created a custom HTTP handler (.ashx) that generates a .NET independent HTML page with a form that POSTs to the URL I want. Basically, a dynamically built HTML page. This way I can dynamically pass my amount value, and then do a normal HTML POST.

You can use the HttpWebRequest and HttpWebResponse classes to achieve this.

For example, to POST to an HTML form that has two fields, username and password you would do something like this:

NameValueCollection nv = new NameValueCollection();
nv.Add("username", "bob");
nv.Add("password", "password");

string method = "POST"; // or GET
string url = "http://www.somesite.com/form.html";

HttpStatusCode httpStatusCode;
string response = SendHTTPRequest(nv, method, url, out httpStatusCode);


public static string SendHTTPRequest(NameValueCollection data, 
         string method, 
         string url, 
         out HttpStatusCode httpStatusCode)
{
  StringBuilder postData = new StringBuilder();
  foreach(string key in data)
  {
    postData.Append(key + "=" + data[key] + "&");
  }

  if(method == "GET" && data.Count > 0)
  {
    url += "?" + postData.ToString();
  }

  HttpWebRequest httpWebRequest = (HttpWebRequest)WebRequest.Create(url);
  httpWebRequest.Method = method;
  httpWebRequest.Accept = "*/*";
  httpWebRequest.ContentType = "application/x-www-form-urlencoded";

  if(method == "POST")
  {
    using(Stream requestStream = httpWebRequest.GetRequestStream())
    {
      using(MemoryStream ms = new MemoryStream())
      {
        using(BinaryWriter bw = new BinaryWriter(ms))
        {
          bw.Write(Encoding.GetEncoding(1252).GetBytes(postData.ToString()));
          ms.WriteTo(requestStream);
        }
      }
    }
  }

  return GetWebResponse(httpWebRequest, out HttpStatusCode httpStatusCode);
}

private static string GetWebResponse(HttpWebRequest httpWebRequest, 
          out HttpStatusCode httpStatusCode)
{
  using(HttpWebResponse httpWebResponse = 
           (HttpWebResponse)httpWebRequest.GetResponse())
  {
    httpStatusCode = httpWebResponse.StatusCode;

    if(httpStatusCode == HttpStatusCode.OK)
    {
      using(Stream responseStream = httpWebResponse.GetResponseStream())
      {
        using(StreamReader responseReader = new StreamReader(responseStream))
        {
          StringBuilder response = new StringBuilder();

          char[] read = new Char[256];
          int count = responseReader.Read(read, 0, 256);

          while(count > 0)
          {
            response.Append(read, 0, count);
            count = responseReader.Read(read, 0, 256);
          }
          responseReader.Close();
          return response.ToString();
        }
      }
    }
    return null;
  }
}

See the HttpWebRequest class. This will allow you to build an HTTP request from scratch, which will allow you to include the data to post to the URL you specify, and get a response back.

You want to use the httpwebrequest class, which has a "method" property that can be set to post. There s an example in the documentation of the property.

http://msdn.microsoft.com/en-us/library/system.net.httpwebrequest.method(VS.71).aspx