But, yeah, that's not exactly a reminder.
Actually, it is. But if you do not heed the reminder to rate the game, you would not heed any reminder to pay for the game. Also, if you do heed the reminder to rate the game (or at least click it away), you might remember at this point, ohh, that was a cool game, I wanted to support the creator.
Also, there are collections. You can have more than one collection.
So anyone wanting to be "reminded", just put the game you just downloaded to a collection named "try before buy", and when you remove the game from that collection and put it into your favorite collection, well, that is your reminder.