We had this semi confirmed by someone who has worked for new car dealerships before. No, they couldn't garentee it beyond a shadow of a doubt, but they did say that the chances are really good that they got a significant discount on the labor, and the recipt was written up to show what I would have paid, if I'd had to pay for this to be fixed myself. In other words, "See what great guys we are? See what we paid to have your car fixed?"
The problem is that I don't really care. I never should have had to deal with this in the first place.
I also mailed the letter. We'll see what happens.