sorry for asking, may not be adequate but have you planned to get revenue somehow?
But we do have a model to generate (modest) revenue, which hopefully will cover the (again, modest) costs: At the end, when the recipient has seen all the letters, s/he can order an actual, physical book with all the text. It's printed on-demand, and we're make a bit of $$$ over what the publisher charges.
But must work as all other online-to-offline modeled businesses. You can contact Springwise.com maybe they'll put in their feed, they like these on/off combinations.
Useful to keep these additional models in mind when thinking of a new business idea