Be a little suspicious

[guado]

Then how does the USPS employee put mail in there? Or is there some kind of semi-standardized USPS lock I can buy for it?

That's what I was thinking. More than likely you have to buy a USPS approved locking mailbox. Kinda like TSA locks for your luggage.

Or this would work!
http://www.steelmailbox.com/html/secure_mail_vault.html