(a) If a bidder requests permission to correct a mistake and clear and convincing evidence establishes both the existence of the mistake and the bid actually intended, the agency head may make a determination permitting the bidder to correct the mistake; provided, that if this correction would result in displacing one or more lower bids, such a determination shall not be made unless the existence of the mistake and the bid actually intended are ascertainable substantially from the invitation and the bid itself.