Selecting an adequate box is the most important single step you can take when shipping an expensive guitar.  Here are some guidelines for finding and evaluating a box—

• The box you select should be a corrugated box, #275 weight with double closure flaps.
• you can likely procure one that meets common carrier standards—and is identical to the one I use for new classical guitar shipments, 50″L x 20″W x 8″D—at a UPS Store (cost around  $15),  but call first to make sure they have one in stock.
• If you can’t find a box at a UPS Store, you can order one from Amazon (search words: “guitar shipping box”) or EcoBox.com; the cost, including shipping, for a single box from one of these suppliers will probably be $32-$33.
• Common carriers (e.g. UPS, Fedex) require that the box be large enough to provide 2″ of space around the guitar; the case itself counts as part of that space.  If you’re shipping a steel-string dreadnaught or jumbo, you will need a 10″D box instead of 8″D.
• DO NOT SCRIMP ON THE BOX; it is the most crucial part of the packaging assembly.  Boxes used by case suppliers for case-only shipments will almost certainly be too flimsy and insubstantial.  You may get lucky and find a free box at a local music store that was used to ship a guitar in a case and that meets common carrier standards; on the other hand, you may end up wasting a lot of time and auto wear-and-tear for nothing.  In most cases, purchasing a new box makes more sense.