You can purchase the return envelope at any post offices including airport post offices.
You can also purchase the return envelope at most convenience stores.
Please ask for a letter pack light (レターパックライト). The price is 370 JPY.
You may request assistance at your hotel receptions
Return Address is as below.

Og inc (Sakura Mobile)
Otakibashi Pacifica Bld. 9th Fl.
7-10-18, Nishishinjuku,
Shinjuku-ku, Tokyo, Japan
160-0023

Please contact us and let us know the incident as well.