CONUS, flat rate box always. You can print/pay for these online because it's flat rate, no weighing game required.
International...this can be a little tricky. Bubble mailers for me usually end up being cheaper, though I have never sent a full set of caps overseas; I am preparing to send a set over and might be able to fit into a bubble mailer but we will have to see. A similar size package would cost about $20 if done in the proper box, could be as low as $12-13 if in a good bubble mailer
edit: adjusted prices