While I commend your generous nature in offering to do this, given that we're talking about $100 or €100 worth of Czech currency, it strikes me as somewhat ridiculous to send currency via the mail (with postage costs and the risk of loss in the mail) just to save a few dollars in currency-conversion fees. I don't know where your home is, but a two-ounce, uninsured letter from the U.S. to Israel now requires $2.71 in postage. That's already 2.71% of $100.

I have to believe that whoever is organizing this trip has given some thought to how and where the teenagers will convert their currency; perhaps Dovster can ask the teenage traveler to inquire about that. Jumping through hoops to save a few dollars on currency conversion of $100 seems a bit much to me.