anyone asking for functionality not already included in a package is asking for custom coding... what if their request slows down, or the development of the 'enhancement' hampers your future development in some way?

what if they call for support questions that you are not prepared to answer, because they have a different distribution of the application than all your other customers?

what if they hound you with calls because the new feature doesn't do 'what they were thinking' ?

all this activity costs you time (=money)... if other users get the benefit of this 'new feature' on next year's release... your requesting client got the functionality sooner and therefore got MORE usage out of the application than their competitors..

Charge em your going consulting rates... if the feature isn't worth that much to them.. it's probably not worth spending the time and effort to develop it :)