You don't agree a price and then accept a shoddy job, the price agreed is for a professional job. However, if it had been me I think I would have walked away the moment he said the old spoiler won't come off "mark free", or at very least asked for and explanation and cost for correction. The fact that you bought a replica spoiler in no way excuses shoddy workmanship which t sounds as if you may have. I had a replica Nismo spoiler fitted to my car last year and the overall finish was blemish free and certainly no cracks of scratches, it sounds to me as if someone not fully skilled or who couldn't be bothered worked on your vehicle. If it is as you say then they should correct the errors at their cost.