Send mail to convenience store?

Is it possible to send my online shopping deliveries to convenience stores in Japan? I'd like to buy some small things from amazon.com / / ebay.

I know that this is possible with Japanese retailers but I'm wondering about overseas retailers.

You can do this to post offices but not to convenience stores (AFAIK).

You can do it with Amazon. They give you this option during checkout. I've used it a few times.

Oh, nevermind, you want to do it with US Amazon. I don't know about that.

How would I get mail (just regular letters) sent to the post office?

Do I have to register / buy a mailbox there or can I just have it sent to the post office then show them my ID to pick it up?

You can send ordinary snail mails to your local JP post branches. But you need to collect it during the time they open.
If You send it to the local mail center(hon-kyoku, or, shuhai-kyoku), you can receive it 24 hrs/365 days from "Yu-yu-madoguchi" counter desk.
It's called "Kyoku-dome" or Poste Restante.
You can use this service without any registration and a free of charge.
JP Post will keep your mail/parcel for 10 days since it has arrived.
You need to bring you ID Card(Passport or driving license etc) to collect it.
If you have a "Hanko" or "Inkan", take with you as you collect a parcel.

Thanks a lot tokyo friend. Do you know if they will also accept A4 sized letters? I want to get some documents mailed in an A4 envelope.

All size can be accepted include A4 size while it can be shipped as a postal service.

Alright thanks again. Mail in Japan is so amazing haha.