Dude, thanks for the heads up, but..........
If the part in question is a fuel hole filler, then yes $20.00 is in the ball park. What were you expecting, I mean we are talking about a slightly concave, round patch piece right?
You should have waited for a response from him, then if you were still unsatisfied come to the board with your complaint. You have to give him the opportunity of making things good first.
Sorry for the lecture, but rookie moves like this give us "newbies" a bad name.