In my experience Arkym also sucks. I bought a rear diffuser that I really wasn't happy with. There were some obvious differences from one side to the other that made for a poor fitment. I expected much better for my M3. I've been installing body kits of some form for almost 20 years on my cars and this piece was worse than the first cheap fiberglass kit I put on a Civic in 94. I too bought from a vendor on here and they tried to be the middle man expressing my concerns but at the end of it Arkym told me that's just the way they are! Really poor. I sold it for $100 and took my loss.
After trying a few other pieces all I have to say is that most of these parts are total shit and not worth the ridiculous jacked up prices. The only parts that I was 100% satisfied with were from Challenge. The difference in quality was notable. Materials, fitment and finish were spot on.