Here's another thing you can do in your app: employ a custom frequency of periodic emails on a per-user basis. It doesn't help to keep emailing someone who never reads your stuff, they either do not need/like your product and ignoring you, or (and this is much worse) they're reporting your mailings as "spam". That, if left untreated, may affect the "spamicity score" of your domain: not good.

I need to do this, thank you @old-gregg.

Post-author here. This was a fun sample app to build. Heroku scheduler made it really easy to deploy the invoice emails on a set schedule. If you have any feedback on ways to improve the app or if you would have done anything differently, let me know.

Awesome write up Roy!

Thank you Chris.

It would probably be better from a economic standpoint just to let your users keep using the email that they are used to using on your site.

This wouldn't work though where the username != email address which is a pretty common situation.

Yeah, you are right.