At a high level, I suspect it could look like this:
a. Purchase is made using your regular store, whether via mobile phone or regular internet browser
b. Confirmation email is sent, as it is today
c. The confirmation email could include a customized link to a special URL (subdomain perhaps?) that you've configured to use a stripped-down template
d. Either way, the customer logs into their My Account area to authenticate, and then clicks the download link
e. Depending on the MIME type of the file being downloaded, you might have to customize the headers sent by the download code or specify a particular protocol to be used for the transfer, etc.
(c) and (e) might require you to do some custom coding to handle custom templates and specialized download needs.
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