You're free to communicate with your former employer in any way you think is appropriate, but there's no reason whatsoever to use certified mail.
Beyond that, it's not entirely clear what's being requested/demanded from you or on what legal or factual basis it is being requested/demanded.
Also, if the other party likely cannot afford an attorney, then you hiring one would put you in a tremendously advantageous position.
Those same two outcomes may result if you engage, so there's not much difference. The other possible outcome if you engage will be that you'll waste your time, which is something you said you'd rather not do.
Small claims court might waste a few hours of your time, but it will waste none of your finances (other than some gas money) if you win.
Well, then there you go.
It will be of little, if any, relevance.
No, "Harold99" does not realize anything logical or sensible. Why in the name of all that is holy do you y'all continue to clutter up thread after thread bickering with such a person?

