Parking at POST Houston

Has anyone used the book parking on POST's website? It pulls up a map with garages and prices and they seem pretty cheap. I saw one for $9 and when you checkout it's like $9+tax. But I was concerned that it's gonna be $9 just to book it and then you have to pay more when you get to the garage? Or is it $9 flat to park?

Also, I'm going to a concert at 713 so I would be parking at night. I heard street parking is free after 6PM so should I just try that instead?

Thanks for any advice!