Ok, so you can't pick out what set you are going to get....BIG DEAL! Most kids do not care what cars they have, they just want them. I ordered one set of Matchbox and one of the Hotwheels and was pleased with both selections I received.
It does say right in the description, items may vary-you just have to read it.
My son loves them and I am very happy too.

Hot wheels are a lot of fun and they are a good value. You typically can't go wrong when you buy them. The hitch in buying them on-line, is that you really don't know what cars you are going to get. If you read the fine print, you'll see that the assortment may vary. You could end up with cars that you don't want or with cars that will not work well on your track layout. I really like Amazon, but for Hot Wheels, make a stop at the local toy store.

Sure, the cars are cool, but hey-It's just 5 more cars. It isn't educational because the only thing it teaches you is wheels roll! And Hot Wheels cars are durable unless you start to bash one with a hammer, which I've done once to a Hot Wheels truck and it was still a little hard.