When you pick up your pocket wifi in Japan, you will receive a prepaid and pre-addressed envelope along with it.
You need this envelope so that you return the pocket wifi at the end of your rental. So if you misplace it, please go to a post office and ask for a "Letter Pack Lite" (or Letter Pack 370) and use it to send your pocket wifi, plus all the accessories, to following address:
SB Frameworks (TFN)
Aomi Center No.1 Bldg 3F1-4
3-4-19 Aomi, Koto-ku, Tokyo
Phone: 050 3196 1160
You can send it at the post office right away, or you can mail the envelope from any postbox in Japan. If you want to return it at the airport on your way home you can, but it’s important to note that there are NO postboxes past the airport security checks. So please be sure to mail it back before then.