A: The way digital goods download works is:

1) A generic copy of the product is created on the server that should be CHMOD (according to the server specifics) so that it can not be downloaded via HTTP protocol.
2) A unique copy of the same product is created on the server with CHMOD that allows download via HTTP. Name of the file is random and can not be reproduced or guessed. Download ticked link is created. Again, it has unique URL. This URL is presented to the customer. Customer uses it to access his unique copy of the product. In the settings you can set up number of downloads to 1, so that upon 1 successful download the customer's copy of the product is deleted automatically. This way you server does not get clogged up. We usually set it up to 2 and than do manual clean up during regular maintenance.