Ok, so let me start off by stating that I have heard this term, but not used specifically for cartography. In English the term refers to all the unnecessary, but enhancing things added to an object. So you could say a truck with all the bells and whistles has all the premium features you have to pay for. Its not just the standard. So did your customer use it in this way? Did they say they wanted all the bells and whistles, or that the maps should have all the bells and whistles? If so, that might be what they are talking about. They just want the fancy stuff. If that's true, just ask for clarification on exactly what they want included. If they specifically meant a style, then I am not sure, and I apologize that I couldn't be of more help.

If it is still unclear perhaps there is a way to show what they said without exposing information you don't want to?