An addendum to all the above helpful replies: Be sure to write down the address before heading off to whatever facility will be shipping your package :)

Also, simply taking the box to your local USPS does work for international shipments as well, but you'll be asked to fill out a customs form declaring what is in the package and how much it's valued at.

Possibly not necessary, but this seems like a post where little nuances are useful to mention :)