would be sad is that you told all that and didn't tell us who the letter is from , so we might be able to help you. Without some information about the letter, as BigA asked, your not getting any help here.
I got a letter from them this morning. They are collecting for a medical lab. I recall paying the lab but will have to go through a few years of bank records to find the proof. It will probably take a couple hours but worth the effort.