You get 5 freebies with the initial test fee. Each school after that is $15. It doesn't matter if you list them before or after you take the test. Though the initial 5 freebies might have to be selected before you take the test if you want them for free...
What you are probably referring to with that second payment is the application itself. That's a different thing altogether. The application through AADSAS is NOT the DAT.