It's in the beginning, under Basic Rules Concepts: Traits, it says: "After [players have used each trait a number of times equal to their screen presence] the player must spend fan mail each additional time he or she wishes to use that trait"

Assuming that you have run out of free uses of your traits, when would you want to  spend a fan mail to use one of your depleted traits?  Because, as I understand it, during any conflict you can spend a fan mail to get an extra card without necessarily involving a trait at all.  So how is using a fan mail to use a depleted trait any better or different, apart from being more limiting to uses of that trait?  IE, wouldn't one always just use fan mail to get the extra card directly instead of using it to activate a depleted trait to *then* get a card?

Am I making any sense?

I am open to hearing from anone that has info on this, but I would eventually like to have Matt's response on this as well.

If you use fanmail to buy extra cards, the Producer has a chance of getting it back as Budget, whereas extra cards from Traits don't do that.

That's the only difference I can think of off-hand.

This thread is referring to the previous edition of the game (hence the reference to dice insead of cards), but I think it should answer your question: http://www.indie-rpgs.com/forum/index.php?topic=13212.0