I am a firmly beleive in the "you break it, you fix it" rule. I have yet to charge anyone for my own mistakes. If it's my own mistake I own up to it, then fix it out of my own pocket. Offten people will respect you more for it and offten give you a tip to help offset the cost and to show good faith in your work.
I also second the "gun doctor feedback" system on here. So far I have just used my normal ASC feedback. I would even suggest having it seperate or avliable to those who sign up to have it (ie the gun smiths that are regulars on this forum).