I think if you do the "express" option, which is the most expensive ($500 pesos or more), you can pay on the day of the appointment, at the same place they take the fingerprints (someone tell me if I'm wrong here). And you get the results later on that day through a web link.
Edit: maybe it's the "muy urgente" option I'm thinking of.
Edit 2: And maybe you can pay this way for all of the options, not just "muy urgente"... just pay at the caja (register) there at the same site.