What happens if the pilot declines or doesn't reply?

The pilot has 24 hours to respond to your request. They, and they alone, decide whether the flight is feasible. Whatever their answer, you risk nothing financially.

If they decline

The pre-authorisation is released immediately. No amount is taken: there is strictly no transaction on your account.

If they don't reply

After 24 hours with no reply, the request closes on its own and the hold is released. It isn't a refusal: the pilot simply didn't have time to decide. You're then free to book another flight.