I had one medical facility sorta do that to me. They basically gave me grief when I called in monthly to make a payment that I could afford. They told me to call collections and pay it. I told them I made the debt with them and I would pay them, not a third party. When I called in to make my last payment, they told me to call the CA. I said read the notes on my account. (They did -- it had been duly noted every month for more than a year.) Then I said your CA has never even contacted me - just take my payment.
The other horrible, horrible one was a hospital. I called them as soon as I knew I would have a copay. The insurance had not even settled at that point. The hospital sent it to collections IMMEDIATELY, but still accept payment. Every CA gets a DV letter. That ends that. Until the next CA letter comes. The hospital still accepts monthly payments. Go figure!
The bottom line is I always pay the OC. I always pay my debt. It may not be as fast as they want for medical, but I DO pay it!
PS - since your debt is paid off, you could do something else. You can call the OC, outraged that you received a collection letter, and ask them what that is all about?? 
They would probably back pedal and say they will pull it back since there is not really anything to collect.