Paint problem, need advice/opinions

I bought my car used w/ 8000 miles from carmax a year and 2 months ago. I've got about 13200 miles on it right now. Well, about 2 weeks ago I was washing my car and I noticed that the paint at one area of the back bumper was peeling/cracking. I've never bumped into anyone, I've never stood on my backbumper/sat on it (I take damn good care of my car ). So I take it to the Ford Body Shop and they tell me that they need to find out if carmax ever messed around w/ the bumper. I call carmax, and they tell me that they had repainted the back bumper, and told me that the reasons for it were that it might've had marks or scratches...Carmax tells me they can't do anything about it, that I'll have to get it fixed w/ my own money. Well, I tell this to Ford, and they tell me that they can't do anyting to the bumper since the paint from carmax voids the warranty on the bumper. I understand that Ford cannot repaint the bumper for free since the warranty was voided. My bumper is getting worst, and it's gona cost a lot of money to get it repainted. I feel like carmax should pay for this and I as well as my dad are gona talk em into doing something about it...What do you guys/gals think, am I just gona waste my time? is there something I can say or do to make this easier?