As simple as this sound. Send him an e-mail and embedd the link.
Dear Customer,
In error our system sent you a link to a text file. Please visit this link and download your product. The link will be active for 36 hours. If you do not download by then please contact us.
http://www.mywebstore.com/customer-name/fileyouneed.zip
We appreciate your consideration and apologize for the error.
Sincerely,
John. Q. Filesender
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Note -- whenever i have to do this for a customer I make a specific folder and put the file in there - i delete it within my time specified.
Hope this helps.
Cheers
JDS
PS -- we also sell software to our customers so i understand the problem.