i've never used a 3D printer before, but have you given the box any thickness to the walls?
try selected all the faces and extruding them inwards to give them a bit of thickness (for software display purposes you'll have to reverse the normals)
that's a "Ch" pronounced as a "K"
Computer skills I should have:
Objective C, C#, Java, MEL. Python, C++, XML, JavaScript, XSLT, HTML, SQL, CSS, FXScript, Clips, SOAR, ActionScript, OpenGL, DirectX
Maya, XSI, Photoshop, AfterEffects, Motion, Illustrator, Flash, Swift3D