The only problem that I have with sending prints outside the country is that I have to go to the post office to do it, due to customs forms. This means that I can only send on a Saturday between 8am and noon, and if I need to do something else then, my print has to sit for another week, or two, or three. Typicaly, this is not a big problem, so I don't really worry about it, but sometimes it makes me miss the deadlines. As to costs, the postcard exchange costs me a lot more than the print exchange does, and since some people are sending %80+ of their cards overseas, I'm not going to complain about it too much.
As far as staggering the exchanges, I could go either way on it. It does take me some time to make a print for the exchange, so having time to do the work without having to print postcards, alt process prints, etc. would help, but I still could go either way.