What happens to the Mets caps that get traded in?

An email from Eric:

I wanted to see what you knew about the Mets Cap Trade promo that’s going on during the Subway Series. It seems like for each game, the first 5,000 people who bring an old cap to the game can trade it in for a new Mets Cap.

At first glance, it sounds good. But I’ve been wondering… what happens to the 15,000 caps the Mets collect over the weekend? Are they donated to a charity or a clothing drive? Are they thrown out?

I actually don’t know.   Anyone?