I have found that booking defaults to "0" when a custom price is configured, but no longer eligible. This has the customer trying to register "0" participants for an event that is past the custom fee.

How can I get the standard fee to default to "1" if the customer early registration fee fee is no longer available (expired)?

See
https://www.certifiedinfosec.com/events/event/48-iso-27001-cyber-security-certification/362-certified-iso-27001-lead-implementer-training-and-certification