Hi AlexLow, welcome to Flyertalk and the Japan Forum.

Before someone better qualified than I for this query comes along, I’m going to stick my oar in as I’m in your exact position for a few weeks every year (non Japanese conversant person in a residential home in Japan).
First of all, the arranging to be home is a hassle, so I try and avoid it and have deliveries made wherever possible to a local convenience store. Much easier and more straightforward to pick up that way.

With JP, having sent lots and lots of parcels within Japan myself, one has to provide a whole lot of personal data including telephone numbers in order to send anything. If you are uncomfortable about going to the Post Office you can try contacting the vendor first to see if they have been given any information.

In your position, I would be going to the Post Office with as much information as possible and making myself known to them. And in as friendly a way as possible. You are going to be living there after all. Organisations such as the post office will usually have group meetings in the mornings, and there will be communication between the staff in the branch and those out delivering (and there will be a really mixed bunch amongst those out delivering, some more conscientious in their roles than others). If you go down with a big smile every time there is a problem and someone uncomfortable with using English has to deal with you each time, in front of all the other clients, they WILL find ways to prevent this. I can practically guarantee it.