The hearing officer, exercising discretion, may elect to conduct any hearing or proceeding under these rules ex parte after a showing that the party who is the subject of the hearing and its representative of record have been notified by certified mail of the pending hearing or proceeding or otherwise served with notice of the hearing. The party who is the subject of the hearing shall at no time be deprived of the opportunity to appear. However, if a party has been served with two written notices of a hearing and fails to appear after each notice, the hearing officer, upon proof of service, shall have the authority to proceed with the scheduled hearing. Proof of service shall consist of a signed certified mail receipt or affidavit.