Shibuya Post Office 渋谷郵便局
Shibuya Post Office is conveniently located near Shibuya station and Shibuya Hikarie. ATM and automated stamp machines are open 24 hours.
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Here is the shibuya post office:website: http://map.japanpost.jp/p/search/dtl/300101041000/
That's great that the post office in japan in opened 24 hours, convenient to mail things, and receive things, unlike in north America: post office aren't opened 24 hours.
This website is in Japanese so you will need to google translate to the language preference of your understanding.