Collection agency
"I called the FTC and the representative referred me to an opinion letter in which it states that a payment can not revoke the statute of limitations. (see the opinion letter on FTC website by Amason)
She advised I mail that letter to this company."
that depends on your state. In some states, simply sending a payment will restart the SOL, but in others, only a written agreement will restart it.
(http://www.creditinfocenter.com)