I don't want to be messing around with "forget my password", or having to remember what email I set up for it.

> some infobox on the app's most attention-requiring screen, and an email telling me I'm gonna be charged

Nope, I no longer log in (because your app was crap, or because I accomplished my goal). Fine I lost $10

If I log in every day or two chances are I want to keep it going. It has to be an out-of-band communication with the ability to cancel in a frictionless way, which means no account hijinks