You can purchase the return envelope at Lawson or any post office, including airport post offices to return the products. 
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.
The 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 about the incident as well.

Below is a photo of the Letter Pack Lite envelope.