Amazon has just launched an online store for downloading software. Currently only TurboTax is available for download but you can expect more software to be delivered this way in the future.
For downloading through the Amazon store you need to install the downloader, after that you can buy to download the software. The software you downloaded will be added to your Amazon media library if you want to download the same at a later date.
You can download a free sample here if you want to test it. [Source]
Good move by Amazon. This will work better if you have a speedier Internet connection.