Q:

Post office reference in movie Ronin

In the movie "Ronin", there is a scene where Sam and the gang have to locate Gregor.  Sam knows a guy locally who can help.  When he approaches his contact he asks him where the nearest post office is.  In response the contact says, "What do you need a post office for?  Because over here, they use them for different things." 
What are these different things he's referring to? 

A:

IMO, they are simply making conversation to seem as if De Niro is a bumbling tourist.  I don't recall the scene exactly, but his contact is then able to pull him aside briefly to have their private conversation and then conclude it by loudly directing De Niro to the post office. 
As for the post office being used for different things, the content of the conversation is quite irrelevant to the plot.  In some countries, the post office can be used for doing additional things (besides posting letters) like paying bills and the like.  Tourists from a country where post offices offer a variety of different services might expect (in this case) French post offices to do the same.  This is presumably why the contact asks (the tourist) De Niro what he actually wants to do at the post office in order to guide him better. 
(I have no idea which services French post offices offer/do not offer.) 

A:

It was just a password exchange.  To an outsider it sounds like ordinary conversation, but to the two people involved in the exchange it allows them to ascertain each other's identities, since only the right people will know the correct words to use.