Draft — have a lawyer review before launch.
1Nothing is refunded
Nothing is refunded, whatever the payment method, including when your account is closed.
2Monthly plans
Plans are paid from your credit each month, and plan credit is added to your balance. A plan lapses to pay-as-you-go when your balance can't cover it. Having a plan charged automatically is your choice; your preset can forbid it.
3Failed and blocked requests
Requests that fail with a server error (5xx) aren't charged. Neither are requests blocked for content the law forbids: they get a 400. There's nothing to refund.
4Closing your account
To keep your unused credit, export it as $5 slips: that closes the account. A remainder under $5 is lost.