Looks like its been pretty much answered, but I'll throw in my two cents.
Many schools will look at your primary app and make sure you make the cut for their school before requesting secondary materials such as an application or just the secondary fee.
Others will require you to send in a secondary payment as soon as you submit your aadsas application. Many schools require add stuff such as a 2x2 photo, 3x5 notecards with contact info, add application info etc. NOTE, Sooner you get this all sent away the better since these schools don't consider you until they recieve them. However, once you e-submit it takes a 4-6 weeks for them to complete your app and send it out. So you have some time before you really need to complete all the seconary stuff that these schools require.
It is best to check the schools site and read specifically what they want, the University of Pittsburgh's site says
"A non-refundable processing fee of $50 (check or money-order) for U.S. citizens and permanent residents, and a fee of $75 (check or money-order) for international applicants (including Canadian applicants) must be mailed to the University two weeks after the AADSAS application is completed."
So feel a little comfort if your feeling pressed for time cuz without your prim app the schools don't need, (or in this case want) your app. But again, better sooner than later.
