I requested via certified mail that my security deposit be returned, but he said that it was forfeited due to a clause in the lease that said it was forfeited if any provision of the lease was violated, in this case not staying the final month.

Don't stop unless they pay everything they owe with certified funds.

Doctor is in Texas but say’s is certified in NY.

If the payment bounces, you apply the late fee (if any) and require payment of the full amount in certified funds (cashiers check or money order).

A great place to start is www.ascsp.org, where you can find certified companies.Also, check out the credentials of the person doing the study.

I know that you also have to either deliver the notice in person or via certified mail.

But when they didn't make up the missing 2 months' rent, I sent them a Certified letter letting them know they were in default, and they needed to pay it by 10/15.

Carefully read what you had them sign, notify them via certified mail (generally, 7 days' notice is needed in AZ), and schedule the HVAC person.

Or send them a certified letter?

For starters, hire an attorney in Cleveland, not Cincinnati.For awhile they were not doing evictions if landlords didn't get their properties lead certified.