You have to have acknowledgement the notice was received. We send ours certified with a return receipt requested.
And before anyone asks. No, receiving the policy would not acknowledge receipt of the notice. All that acknowledges is you told them they needed flood insurance and they got it but it does not acknowledge you sent the notice. As for as the examiner is concerned you told them over the telephone they needed the insurance which does not meet the notification requirements.
The opinions expressed are mine and they are not to be taken as legal advice.