I would like to find a company or service that offers to accept a
parcel (no more than 2kg)in Japan, ie. the address must be in Japan
and then forward this parcel onto an international address.

I've noticed quite a few companies that offer a service like this, but
it involves signing up for an extended period of time (6 or 12
months). I am after a one-time parcel forwarding service

I would like this service to be as simple as possible, and ideally not
involving auction sites