IPA: /ˈpoʊstˌbɒks/
KK: /ˈpoʊstˌbɑks/
A public box where people can drop off letters and packages to be sent by mail.
I put the letter in the postbox on the corner of the street.
Postbox → It is formed from "post" (from Latin *postis*, meaning door or gate, and later used to refer to mail) and "box" (from Old English *box*, meaning a container). The word "postbox" refers to a container for mail, typically placed at a location for collecting letters.
Think of a 'box' that is specifically for 'post' or mail — that's why a postbox is where you drop off your letters.
No commonly confused words.