I sent a pay for delete letter by certified mail along with return receipt requested to my credior,?

I sent a pay for delete letter by certified mail along with return receipt requested to my credior, in the letter I gave them 15 days to respond or else I would take further action. 20 business days have ped and they still the have not responded. what can I do. If 30 days p can I dispute this debt. Is this pay to delete letter considered a debt validation letter?