OK, let me see if I understand this Grand Prix thing because it looks like you can get money for achievements.
First, you have to play a game for at least 30 minutes for it to show up on the games list. Rarer achievements should count for more so aim for those games. Also, the game might not show up until the next day.
The amount of points you can claim per day is up at the top near the driver dashboard. Purchases you have made on Steam since the Lunar New Years sale plus leftover Lunar New Years points count toward that max (I presume key sellers count for dink). So go down to your achievement games and claim those, then go to the top area and cash in the points. For whatever reason, you can only cash in 1000 at a time so you might have to do that a lot.
When you get 15,000 points used, you can go to the Pit Stop and claim the prize to get $5 off your next purchase. However, that $5 is NOT a coupon. It looks like you have to make the purchase in the Steam sale.
So, what you need to do is buy your games one at a time. You might need to buy one just to get your max up. Then you can buy a game every time you get up to the 15K point and can redeem a "bonus saving mode."
P.S. I'm completely ignoring the race winner rewards.