So when we tried to do download codes for the Kristin Hersh 'Crooked' art book the issue was that the plant couldn't actually insert things into the book before shrink-wrapping. Probably a low hurdle to overcome, but our lame quick-fix was to do the "first word on page x, last word on page y" as a password to get all the download content. But the situation was a little different. I like Erin's idea of the code on a receipt (a lot) but worry it would give control to the retail chains, not to the publisher or author...