>We already create zip files of the data, so that is solved. The question really is about how to upload and download files to space rented from Amazon.
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>Another key issue is how to so organize things so that no customer can download data belonging to another customer.

If you do business in the cloud with such business, I am pretty sure they are offering a good security and are responsible for doing so. I would mostly worry if you would start uploading your content to an unknown provider. Let us know about your findings. It is always interesting to here good advices of satisfied customers using such service.