Perhaps there is room for the following:

(Please dont kill me for the off-the-cuff idea:)

A per-login / frequency of use model;

You agree to a MAXIMUM of $100/year as WELL AS a max per month that may be charged (== to $100/12 max) -- but the idea is that if you skip a month or some amount of time you are not charged....

The usage is based on certain amount of time-in-app or somesuch....

you get the idea.

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Anyway, as someone who has debilitating ADHD I really want to use this... but whilst not working, I can't pre-commit $100 to something that I may have too bad a case of ADHD to adopt on a regular.