This is an interesting marketing fact. Apparently, general sales are less effective than specifically targeted sales.
This naturally leads to an interesting idea. What if by achieving certain achievements in game, you could earn discounts on certain paint jobs? Someone might not buy a paint job if everyone had a discount on it, but if they themselves had earned that specific discount through some achievement or action, like hauling 10,000 or 100,000 tons, buying that paint job might suddenly become significantly more appealing.