This is the 928th most frequent Japanese word.
郵便
"郵便" means "mail" or "post."
Here, "郵便局" (post office) incorporates the word "郵便" to specify a facility managing postal services.
郵便局に行って手紙を出しました。
I went to the post office and sent a letter.
"郵便物" (mail items) includes the word "郵便", indicating objects delivered by postal service.
郵便物がまだ届いていないようです。
The mail hasn't arrived yet.
In this sentence, "郵便ポスト" (postbox) uses "郵便" to signify its role in the mail system.