> anything popular that used this system seems like it'd be pretty inundated with fraud

I coined "micropayments means microfraud"; I would expect this to have similar situations to the AWS mystery bill problem, but on a tiny scale. If you can charge customers without their confirmation it's easy to run up bills. And of course the amounts are so tiny you can't afford dispute resolution.

Yes, merchant abuse is a risk. What we do and plan to do:

  - Each merchant requires an OAuth grant, and customers can revoke it at any time.
  - A customer ledger shows what, when, and how much each merchant charged. This can be shown in the customer's dashboard and monthly statement emails.
  - Customers have account-level spending caps to limit exposure. We will add per-merchant caps.
  - If patterns look off or we get complaints, we can pause new charges and review.