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I'd go down to the CU and pay it but get the letter then and there.
If it were me (and I know a three hour drive is a lot) I'd drive to make that happen. After learning what I have about the damage it can do for 7 years, I'd probably kick myself in the butt for not doing it. I personally was burned by CA (Midland) that promised to remove a collection upon payment. But I didn't get it in writing and they reported it as "Closed - Paid", but never removed. I lodged several complaints against them asking them to review phone records, but was never able to win the battle. They said under no circumstances would a rep have ever said that and I honestly had nothing to prove otherwise. Lesson learned.
I think they are trying to set you up to be played. What do they lose by sending you a letter that says they will delete if you pay? If they offer PFD which in this case they do, i will request the person sends an email. you can actually offer to draft it for them if they are being lazy about it.
If they dont want to do it over email which is very easy, i doubt if they will do it in branch.
I think your next steps to be get the CU call the debt back in house and then you can deal with them. CAs can be very difficult to deal with