Totally agree, and if they could see the damage after removal of the original spoiler, which is a pig to remove, why didn't they ring you and ask you to come in and see it.
That would then have been the point for negotiation, as far as the finish on the new spoiler, copy or not if it needed more prep, then once again, whats wrong with a phone call.
Every business has glitches shall we say. It's how you deal with your customers and put things right. Everone hears about 1 bad job but nobody hears about all the good ones.