I think your solution is appropriate. Send it certified or registered mail, or overnight mail (e.g. FedEx) which requires a disgnature, or personal delivery by a messenger service to avoid future controversy as to whether it was received. If it was filed with the County Recorder's Office (required for Powers which permit real estate transactions, you should also file your revocation with the County Recorder's Office.
The above is general legal and business analysis. It is not "legal advice" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
Everything previously stated is correct, I would add I would also give a copy of the revocation to all financial (banks, credit cards, etc.) and medical institutions that I regularly deal with so that they have actual notice of the revocation.