Hi Akum

In order to send feeds to Amazon you need to include an XML body and ContentMD5Value in the Amazon request.   This first function prepares the XML body, it assumes you are looping through all the products you have in Salesforce that you want to create or update in Amazon.  You need to pass it your seller id and a list of the products you want to send to Amazon.  And update it with your own values.
 

//Prepare the feed 
private String prepare_body(List<Products__c> products, String sellerId){
        
       String xmlBody = '<?xml version="1.0"?>' + 
                     '<AmazonEnvelope xmlns:xsi=http://www.w3.org/2001/XMLSchema-
                     instance xsi:noNamespaceSchemaLocation="amzn-envelope.xsd">'+
	               '<Header>' +
            		'<DocumentVersion>1.01</DocumentVersion>' +
            		'<MerchantIdentifier>' + sellerId + '</MerchantIdentifier>' +
            	      '</Header>' +
            	      '<MessageType>OrderAdjustment</MessageType>' +
            	      '<Message>' +
            		  '<MessageID>1</MessageID>' +
                       '<OperationType>Update</OperationType>' ;
     
        // replace all the values here with your own
        For(Product__c product : products){
    	xmlBody  += '<Product>' +
      			'<SKU>product.SKU__c</SKU>' +
      			'<StandardProductID>' +
        			'<Type>ASIN</Type>' +
        			'<Value>product.ASIN__c</Value>' +
      			'</StandardProductID>' +
      			'<ProductTaxCode>product.TAXCODE__c</ProductTaxCode>' +
      			'<DescriptionData>' +
        			'<Title>product.TITLE__c</Title>' +
        			'<Brand>product.BRAND__c</Brand>' +
        			'<Description>product.DESCRIPTION__c</Description>' +
        			'<BulletPoint>product.BULLETPOINT1__c</BulletPoint>' +
        			'<BulletPoint>product.BULLETPOINT2__c</BulletPoint>' +
        			'<MSRP currency="USD"> product.MSRP__c</MSRP>' +
                    '<Manufacturer>product.MANUFACTURER__c</Manufacturer>' +
        				'<ItemType>example-item-type</ItemType>' +
      			'</DescriptionData>' +
      			'<ProductData>' +
        			'<Health>' +
          		          '<ProductType>' +
            				'<HealthMisc>' +										 
                               '<Ingredients>product.INGREDIENTS__c</Ingredients>' +
              					'<Directions>product.DIRECTIONS__c</Directions>' +
            				'</HealthMisc>' +
          		           '</ProductType>' +
        			  '</Health>' +
     			 '</ProductData>' +
    		'</Product>';
            }
            xmlBody += '</Message>' +
                                 '</AmazonEnvelope>' ;

            return xmlBody;
    }

Then use this XML Body to get the ContentMD5Value:

private String get_contentmd5(String xmlBody){
        	Blob targetBlob = Blob.valueOf(xmlBody);
        	Blob hash = Crypto.generateDigest('MD5', targetBlob);    
        	String contentMD5 = EncodingUtil.base64Encode(hash);
        	contentMD5 =  EncodingUtil.URLENCODE(contentMD5,'UTF-8');
        
        	return contentMD5;
    }

Then send the request to Amazon, you will need to prepare a signature to do this.

private void send_feed_to_amazon(String xmlBody, String ContentMD5){

         String timestamp = datetime.now().formatGMT('yyyy-MM-
           dd\'T\'HH:mm:ss.SSS\'Z\'');
        
        //Construct a query string with the query information
        String queryString = 'AWSAccessKeyId=' + accessKey + 
            '&Action=SubmitFeed'+
            '&FeedType=_POST_PRODUCT_DATA_'+
            '&MarketplaceId.Id.1=' + MarketplaceId  +
            '&SellerId=' + SellerId +
            '&SignatureMethod=' + SignatureMethod  +
            '&SignatureVersion=' + SignatureVersion  +
            '&Timestamp=' + timestamp  +
            '&Version=' + version;

        String stringtoSign = 'POST' + '\n' +
            'mws.amazonservices.co.uk' + '\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');


      //Create a new request
       HttpRequest request = new HttpRequest();    
        
       request.setEndpoint(endpoint +'?AWSAccessKeyId=' + accessKey +
            '&Action=SubmitFeed'+
            '&FeedType=_POST_PRODUCT_DATA_'+
            '&ContentMD5Value=' + ContentMD5+
            '&SellerId=' + sellerId +
            '&SignatureVersion=2' +
            '&Timestamp=' + timestamp +
            '&Version=' + version +
            '&Signature=' + signature +
            '&SignatureMethod='+ signatureMethod +
            '&MarketplaceIdList.Id.1=' + marketplaceId);
        
        request.setMethod('POST');
        request.setBody(xmlBody);
        
        Http http = new Http();
        try {
            HttpResponse response = http.send(request);
            System.debug(response.getStatus());
            System.debug(response.getStatusCode());
            System.debug(response.getBody());
        } 
        catch(System.CalloutException e) {
            System.debug(e);
        }
}

This should return your FeedSubmissionId and you can use that to check the status and confirm that the feed was accepted.
See http://docs.developer.amazonservices.com/en_UK/feeds/Feeds_GetFeedSubmissionResult.html

I hope this helps.
Gráinne