Try first class mail. In court, first class mail is deemed delivered. Certified mail is questionable. If the receipt is signed by a different person than the addressee, it can be thrown out. It is not unusual that certified mail is not picked up by the addressee.
If you are not comfortable with first class only, you can ask for a tracking number from the post office. They will notify you once the letter has been delivered.
This bit of advice was given to me by my law firm when I was a HR Director.
Excellent idea about the "tracking number". I remember on a different issue, I sent mail certified return receipt. It was never picked up and then returned to me. I called and asked why they didn't pick it up, they said they never accept a certified letter. I thought strange. When I receive notice via certified it usually means there's something regarding zoning that's going on in the neighborhood and it's important. I think the tracking idea was good.