You can mail them anywhere. It is just cheaper closer to the origin location and it gets more expensive the farther you ship them. They are a great deal if you ship within a few shipping zones of your location. Beyond that, the standard flat rate boxes may be a better deal. Also, the regional rate boxes have weight restrictions within the United States. The standard flat rate boxes have no weight restrictions if you ship domestically, but they do have weight limits if you ship internationally.
Go to the Post Office website and you can easily calculate postage and make a shipping box decision.