Increase in costs *after* work has been done - opinions, please!

[Neeek]

I'm using a cars as a kind of metaphor here - I have the exact same situation but completely unrelated to cars, so opinions, please!

You entrust some work to your mechanic on your car. He quotes, you agree to go ahead based on that quote (which was priovided in writing and you signed-off on) and the work is done satisfactorily.

After the work has been completed, the mechanic sends you a letter stating that his original quote was a little off the mark, and he would be most appreciative if you could pay an additional 30% on top of the original quote (which you paid some time ago) to cover the extra costs he incurred.

Now, as far as I'm concerned, I've paid for the services, I'm satisfied with them and the deal has been done - that's it. Surely if costs were exceeding the original quote, then it was the "mechanic's" responsibility to make me aware of the potential rise in overall costs BEFORE the work was completed - right?

My initial reaction is to tell him to get stuffed. Thoughts, please.