The question is:
Can you drop off a parcel to the PO BOX if it's local, rather than sending it?
I mean, walk into the PO Box Office where their PO BOX is and giving it to them to put into there PO BOX without sending it, or posting it…
I'd ask Aust Post but I'd have to wait to tomorrow; I may get an answer here :)
I suspect Auspost is not in the business of being a free parcel drop service (even though the recipient is paying for box rental) so I think they will still want to charge you the minimum postage.