I think the most scalable method for you to achieve this is using AWS Elastic Map Reduce and Data pipeline.
The architecture is this way:
You will use Data pipeline to configure S3 as an input data node, then EC2 with pig/hive scripts to do the required processing to send the data to SFTP. Pig is extendable to have a custom UDF (user defined function) to send data to SFTP. Then you can setup this pipeline to run at a periodical interval. Having said this this, it requires quite some reading to achieve all these - But a good skill to achieve if you for see future data transformation needs.
Start reading from here:
http://aws.typepad.com/aws/2012/11/the-new-amazon-data-pipeline.html
Similar method can be used for Taking periodic backup of DynamoDB to S3, Reading files from FTP servers, processing and moving to say S3/RDS etc.