If they send a tech out due to an unsuccessful self-install, you are charged a fee. This is the same as if you had paid a fee for a tech to do the install. Sounds like the employee gave you bad info though.
I've asked a Comcast employee to help you. You should expect a reply in this thread.
Hello Lei2. I can help in reviewing your bill for the technician visit fee you were charged. nerdburg is correct, we do charge a "failed self install" fee. So that I can begin the review, please send me a private message and include your full name, service address, and account number so I can assist you.