Okay, so i went on my first long bike ride (about 50 miles) and i was wearing a backpack the whole way. Man the sweat mark i had was quite emberassing. So I was wondering, whats the best, preferably cheapest rack i can mount on the back of my bike, just so strap my backpack to with a bungee net.

I know at the target by my house, they have this but it seems like it might be kind of bad because its mounted to the seat post.

One of my LBS's has this but thats farther away.

So, are seat mounted on bike racks a bad idea?