Where to send offer letter?

The OC is usually the best one to work with. But it really depends who currently owns the debt, and what their policy is.

If the original creditor still owns the debt but has just retained the CA to collect, then you should try settling directly with the OC. Call them and see, then get it all in writing before you pay.

However, some OC have a policy that they won't deal with you after sent for collections, and they will refer you to the CA. If that happens, try telling them you won't work with the CA and maybe they'll budge.

BUT if the account was sold, then the OC actually has nothing to do with the account anymore and you will have to work with the CA in order to pay it off.