I have my 2011 duramax, 60K miles in the second time in a month for the cleaning exhaust filter message coming on and not going out. First time they said it needed a manual regen. Cost me $300. Don't ...
OP, I would go back and see if this charge is correct.
The problem is, who do you go to? The service advisor? I never met one that didn't lie more than a politician. :R He is going to tell you that the charges are correct.
You might open your OM and read up on emission warrantee for you vehicle. That way when you go in you might have a little ammo.