Here's why:
* Common nouns refer to general things, not specific ones.
* "Postal service" refers to the system of delivering mail, not a specific organization like the USPS.
Examples:
* Common Noun: The postal service is vital for communication.
* Proper Noun: The United States Postal Service (USPS) is the primary postal service in the United States.
Let me know if you have any other grammar questions!