I'm constantly walking into the courthouse to get something verified and stamped by the sheriffs office for my beloved CASA (I think the 'S' is for Scaremongering!!) and they are happy to oblige at no charge and I haven't spent any money with them (at least no bribes or bail that I recall!!).
I understand that people deserve to be rewarded for their time and that this exercise is more complicated than just certifying a document but the people in question have spent a considerable amount of cash with one organisation.
It's reasonable to assume that 99% of the Indian students in question (as we are only talking about the DGCA here) have completed all of there training within the one place on an integrated programme that surely an hour of someones time is no unreasonable request at no cost. If however you're having to do the same for people that have only paid for a couple hours of training with you then that's different.......perhaps the rule you could take is no charge for the 'full time' students, as in the ones that complete all of their qualifications with the one establishment??