Let me clarify a few things here. The guide is as much about selfish desires as anything else. Who here (except Rabbi Poxxy) doesn't want a nice, pleasant transaction that ends with all parties happy. I know of no other business where the amount to be paid is not clear from the beginning.

Hello Mr Wal-Mart. I'm going to take this food home and then pay you depending on how delicious it is!
Hello Mr Spielberg. I'm going to watch your movie and then pay you depending on how enjoyable it was!
Hello Mr Decorator. You can paint my house and then I'll pay you depending on how much I like it!