I'm trying to integrate amazon with salesforce since day's but got stucked. Although i'm providing the correct "Access Key", "Security Key", "Token" etc, the error keeps on popping up.
Can anyone help me to figure out where i'm wrong?

List<AmazonMWS__c> amazonList = AmazonMWS__c.getall().values();
    	
    	//Current time in GMT ISO 8601
        String timestamp = datetime.now().formatGMT('yyyy-MM-dd\'T\'HH:mm:ss\'Z\'');
        timestamp = EncodingUtil.urlEncode(timestamp,'UTF-8');
        
        //Amazon Variables
        String action = 'ListOrders';
        String version = '2013-09-01';
        String signatureVersion = '2';
        String signatureMethod = 'HmacSHA256';
        String marketplaceId= amazonList[0].Marketplace_ID__c;
        String marketplaceToken = amazonList[0].MWSAuthToken__c;
        String fulfillmentChannel = 'AFN';
        String sellerId = amazonList[0].Seller_ID__c;
        String endpoint = 'https://mws.amazonservices.com/Orders/2013-09-01';
        String accessKey = amazonList[0].AWSAccessKey__c;
        String amazonSecretKey = amazonList[0].Secret_Key__c;
        
        //Construct a query string with the query information
        String queryString = 'AWSAccessKeyId=' + accessKey + 
				            '&Action=' + action  +
				            '&SellerId=' + sellerId +
				            '&MWSAuthToken=' + marketplaceToken  +
				            '&SignatureVersion=' + signatureVersion  +
				            '&Timestamp=' + timestamp  +
				            '&Version=' + version +
				            '&SignatureMethod=' + signatureMethod  +
				            '&CreatedAfter=2016-09-28T18%3A30%3A00Z' +
				            '&MarketplaceId.Id.1=' + marketplaceId  ;
        
        String stringtoSign = 'POST' + '\n' +
				            'mws.amazonservices.com' + '\n' +
				            '/Orders/2013-09-01' + '\n' +
				            queryString;
        
        //Covert query string to signature using Amazon secret key as key
        Blob mac = Crypto.generateMac('HMacSHA256', blob.valueof(stringtoSign),blob.valueof(amazonSecretKey));
        String signature = EncodingUtil.base64Encode(mac);
        signature = EncodingUtil.urlEncode(signature,'UTF-8');

        system.debug('signature '+signature);
    	
    	HttpRequest req = new HttpRequest();    
        
        req.setEndpoint(endpoint +'?AWSAccessKeyId=' + accessKey +
			            '&Action=' + action +
			            '&SellerId=' + sellerId +
			            '&MWSAuthToken=' + marketplaceToken  +
			            '&SignatureVersion=2' +
			            '&Timestamp=' + timestamp +
			            '&Version=' + version+
			            '&Signature=' + signature +
			            '&SignatureMethod='+ signatureMethod+
			            '&CreatedAfter=2016-09-28T18%3A30%3A00Z' +
			            '&MarketplaceId.Id.1=' + marketplaceId);
        
       req.setMethod('POST');
       system.debug('req'+req);
       Http http = new Http();
       HttpResponse res = http.send(req);
       system.debug(res.getStatus());
       system.debug(res.getStatusCode());
       system.debug(res.getBody());