Didn’t see any in the Post Office or 7-11/Family Mart. I want to send some stuff back to the US but I don’t know where you buy standard boxes/containers for mailing packages.
Post office behind the desk. You need to ask for a box. Some larger post offices even have packing material for free.
Convenience stores sell them too. But you need to ask.
Or find a T-Cat drop off point near where you live.