Internationally can get tricky. The medium flat rate box is over $40 to send, so you're better off doing a box first class if the weight is low enough (but you have to be careful about dimensions, I think it has to be under 36 linear inches-- so 2 x 10 x 12 would be ok). But then you have to think about time constraints, since first class doesn't come with the same speed. The flat rate padded envelope has become my happy medium, as long as there is nothing breakable. You get the priority speed if you need it, and you get the lower rate.
The first time I mailed stuff internationally, I did a lot of research because I was really shocked by the medium box flat rate shipping, which I had planned to use. I still look up the best way to ship online before I send a package.