https://firebase.google.com/docs/projects/billing/firebase-p...
I am not sure how they can do that, but cannot let people set their own limits on their paid plans.
Because it's a free plan, the delay between 'limits reached' and actual shutdown only incurs the cost of providing the service during that brief period, not the potential liability of overcharging that might exist on a paid plan.
Or write a disclaimer that the billing cap doesn't necessarily cut off at exactly that amount and that there might be an overcharge.
I am pretty sure most people would be okay with either of these options, we didn't need a perfect system, just one that works well enough