Unable to call Amazon MWS API 'GetLowestPricedOffersForSKU' Post call from Google Apps Script.

Question

We can make most of the Amazon MWS API calls using GET method but a few (like GetLowestPricedOffersForSKU) really needs to be through POST call.

I have successfully called Amazon MWS API GET calls with proper signature from Google Apps Script but Post call is not working.

I am getting this error: 'Timestamp 2018-06-01T09%3A11%3A52.000Z must be in ISO8601 format'.

If I don't encode timestamp then I am getting 'Incorrect Signature' error.

Notes:

1. I am not passing querystring. If I do that I get an error message mentioned here: GetLowestPricedOffersForSKU failed processing arguments

2. I am adding all the parameters in the payload.

   var options = { 'method' : 'POST', 'contentType': 'application/x-www-form-urlencoded', 'payload' : payload, 'muteHttpExceptions':true };

   response=UrlFetchApp.fetch(finalURL,options);

3. The Signature calculation process is same as other API calls which are working perfectly fine.

Please let me know if anyone has implemented Amazon MWS Post method call using Google Apps Script.