I wouldn't expect top notch, I helped a forum member make a box to return a carbon one a few years ago as it had sunk in the middle, I also saw another forum member who had one in FRP and it was wavey as hell. For the money they are ok if you don't get the issues mentioned above but its going to be cheap chinese fibre glass, which doesn't really matter too much with spoilers as long as no one sits on it. Its also pretty likely to fit as its not butting up to anything.
I could sell this type of product but it can often be a ball ache, I would rather sell people a 3d carbon Nismo style wing which is the best quality FRP I have ever seen, I sold a GT86 3d carbon body kit before xmas and the customer could not believe the quality. But it comes at a premium, more than 3 times the price of one of these so I understand why people by cheaper products.