Thank you! I hadn't chimed in earlier as I've never done this with actual luggage (although I knew one could have luggage forwarded to a takkyubin centre).

I take my kid to Tokyo every year so she can spend time with her granddad, means we sleep on futon at his apartment. Am not comfortable getting things sent to grandad as it forces him to be at home during whole time windows to receive the packages. So I select a Family Mart (convenience store) round the corner from him and have the items sent there. Means I can collect the items personally any moment, day or night, that suits me.
I don't read/write Japanese so I use Amazon jp in English and run a few things through translation software. I'd recommend getting a Japanese friend to help with this "concierge service" before you arrive in Japan. I've also been extremely fortunate to have kind and able staff at the local Family Mart assisting me with the pick up procedure (involves a touch screen interface).
To summarise: A little tricky, but doable.

Actually SENDING luggage - as SQ319 describes - should be a piece of cake.