Post
noun
Mail (or post) has convenience because it is sent for you in the mail (or the post!).
郵便 originally referred to the postal system, but eventually also came to refer to mail itself as a shorthand for 郵便物.
This is a jukugo word, which usually means on'yomi readings from the kanji. The reading for 便 here (びん) isn't the on'yomi you learned with the kanji though, so here's a mnemonic to help you out:
You get a lot of junk mail, so you throw it in the waste bin (びん). The waste bin is where that mail goes.
おくさーん、郵便ですよー！
Madam, we have mail for you!
そこには明るい赤色の郵便ポストがありました。
There was a bright red mailbox there.
郵便番号は何番ですか？
What is the postal code?
郵便業においては、効率とスピードが全てだ。
Efficiency and speed are everything in mail service.