I guess you will have to take this up with the forwarding company. Both postal codes are in Koto-ku, Tokyo. Most likely 136-0075 is where the company is located. 136-8799 is a post office in Koto-ku:
Find local businesses, view maps and get driving directions in Google Maps.
I assume your shipment was deposited at that post office. Anyhow, all this sounds a little suspect.