Good idea with the watermark.
Have you played with the envelope tool yet? "Make with mesh" or "Make with top object?"
I'd use it to distort the "78" to lay it down on the panel and into perspective.
Another trick to give the car some weight and definition is to use thicker strokes on the bottom of the shapes to work as a shadow line.
Also, the top inside of the wheel will reflect the ground, so that might give it a touch more zing on the wheels.
...but im not sure people are willing to pay nearly 800$ for a design.
Sadly, it's not like they used to... but it's not completely out of line. I've seen it for more, too. Personally, I've been between $400-$800 for work of similar type-automotive style drawings/25-30hours. Most of falling in the mid-range working for a middle man.
You're more competent than most I've seen - a few tweeks and you could go from really, really, really good to outeffenstanding. The background treatment on your last one was great! That's what will put you over the top.
You've got a good eye for detail