郵便局
ゆうびんきょく · yuubinkyoku · JLPT N5 · noun
post office
🏣📮 郵便局 = mail (郵) + delivery/convenience (便) + bureau (局)
郵便 (yuubin) means mail: 郵 is post, 便 is delivery or convenience. 局 (kyoku) is a bureau or office, the same kyoku as in 薬局 (yakkyoku, pharmacy) and テレビ局 (TV station). So yuubin + kyoku = the mail bureau.
便 — person 亻 + change 更
A semantic compound: 亻(person) beside 更 (to change/replace). The original sense was a person making things easier or more convenient by adjusting them. From 'convenient' it extended to 'then/thereupon' (biàn) and to bodily 'discharge' (as in 大便).
局 — 尸 (body/shelter) + 句 (phrase, phonetic)
局 originally depicted a confined space marked off by a boundary, with 尸 suggesting an enclosed shelter and 句 hinting at the sound. From 'enclosed area' it extended to mean a chessboard square, then a defined situation, office, or bureau. Modern senses like 'department' and 'situation' grew from this idea of a bounded scope.