The airlines have become very careful about what they transport, especially since 9/11.  The airlines will assume any package markings refer to the actual contents. When reusing a box for mailable items, be sure to completely mark out any ORM-D markings,  hazmat diamonds, and other indications on the box such as "alcohol" or "bleach". Your package will be refused or returned at the airport and delay delivery for you.  Also cover or mark out any old addresses and UPC/barcodes. That way your mailpiece is  not misrouted when automated equipment picks up a wrong number and inputs it as a zip code.