HttpWebRequest/DevDefined - QBO Static Base URL

public string GetQboInvoices()
{
    StringBuilder requestXML = new StringBuilder();
    StringBuilder responseXML = new StringBuilder();
    string realmId = "<<SetRealmId>>";

    HttpWebRequest httpWebRequest = WebRequest.Create("https://qbo.sbfinance.intuit.com/resource/invoices/v2/" + realmId) as HttpWebRequest;
    httpWebRequest.Method = "POST";
    httpWebRequest.ContentType = "application/x-www-form-urlencoded";
    httpWebRequest.Headers.Add("Authorization", GetDevDefinedOAuthHeader(httpWebRequest));
    requestXML.Append("PageNum=1&ResultsPerPage=100");
    UTF8Encoding encoding = new UTF8Encoding();
    byte[] content = encoding.GetBytes(requestXML.ToString());
    using (var stream = httpWebRequest.GetRequestStream())
    {
        stream.Write(content, 0, content.Length);
    }
    HttpWebResponse httpWebResponse = httpWebRequest.GetResponse() as HttpWebResponse;
    using (Stream data = httpWebResponse.GetResponseStream())
    {
        return new StreamReader(data).ReadToEnd();
    }
}

protected string GetDevDefinedOAuthHeader(HttpWebRequest webRequest)
{

    OAuthConsumerContext consumerContext = new OAuthConsumerContext
    {
        ConsumerKey = consumerKey,
        ConsumerSecret = consumerSecret,
        SignatureMethod = SignatureMethod.HmacSha1

    };

    consumerContext.UseHeaderForOAuthParameters = true;

    OAuthSession oSession = new OAuthSession(consumerContext, "https://www.example.com",
                            "https://www.example.com",
                            "https://www.example.com");


    oSession.AccessToken = new TokenBase
    {
        Token = accessToken,
        ConsumerKey = consumerKey,
        TokenSecret = accessTokenSecret
    };

    IConsumerRequest consumerRequest = oSession.Request();
    consumerRequest = ConsumerRequestExtensions.ForMethod(consumerRequest, webRequest.Method);
    consumerRequest = ConsumerRequestExtensions.ForUri(consumerRequest, webRequest.RequestUri);
    if (webRequest.Headers.Count > 0)
    {
        ConsumerRequestExtensions.AlterContext(consumerRequest, context => context.Headers = webRequest.Headers);
        if (webRequest.Headers[HttpRequestHeader.ContentType] == "application/x-www-form-urlencoded")
        {
            Dictionary<string, string> formParameters = new Dictionary<string, string>{{"PageNum", "1"}, {"ResultsPerPage", "100"}};
            consumerRequest = consumerRequest.WithFormParameters(formParameters);
        }
    }

    consumerRequest = consumerRequest.SignWithToken();
    return consumerRequest.Context.GenerateOAuthParametersForHeader();
}