How billing works
Payments through Stripe, and what happens if a charge fails.
Billing runs through Stripe: we neither see nor store your card details. You can change plan or cancel whenever you want.
If a charge fails we email you, and you get a few days of grace before the plan drops. A bank can decline a charge for a thousand reasons, and cutting you off the same day would punish you for something you are probably about to fix.
Nothing is deleted when you downgrade. Whatever exceeds the limit is deactivated, and comes back when you upgrade.