FILE THIS ONE UNDER "CLUELESS"
I was born in 1957 and grew up in a hamlet where everybody had a mailbox located at the curb of their property. The mailman drove a mail truck and was a neighbor who lived around the corner from us.
During the 1960's, the little hardware store just down the street was also the 'post office' for hamlet dwellers, My father bought postage stamps (a nickel a piece) as we needed them and mailed whatever had to be mailed. It was simple and I never gave the United States Postal Service a second thought.
Somewhere along the line, I went to school with kids whose addresses were other than a street name and number. They would be, for example, "Rural Route, PO Box 112....." I was young and inquisitive, so I asked about it and was told they live 'in the country'. It wasn't as if I lived in the city! But, okay, if that's what I was told, who was I to argue?
When the 1970's arrived, two of my brothers perfected the art of intercepting mail sent to my father from their schools. Well now! Dad was none too happy about this development! So he did what any quick thinking parent would do, he got himself a PO Box and had all the mail addressed to him mailed to the new address. Score: Dad 1, Brothers 0.
When I was 16 years old (that would be in 1973, in case you're keeping track), I accompanied my father on morning errands one Saturday. One of those errands was stopping at city post office to pick up my father's mail.
The city of Poughkeepsie's post office building was a grand and glorious old structure. Rather than waiting in the car, I decided to go inside and see what a post office was like. (Remember, I'd never been in one before.)

Up the stairs, through the doors into a great big lobby we went. The ceilings were high, there were multiple lines open for customers and there were huge, long walls that looked like this:
I pointed to the wall and asked my father, "What's all that?"
Dad's face was laced with skepticism and perhaps a bit of impatience. "Those are post office boxes," he replied.
"You mean they're really boxes?!!" I said, incredulously (and a bit too loudly).
My father started to laugh. "What did you think a PO Box was?"
I was stunned! It was so basic and I was 16 years old. I thought I knew EVERYTHING! "I thought a PO Box was just an address, like a rural route address. I had no idea it was actually a box!"
It was cozy, cool and damp under the rock where I lived.... :)
Always looking forward, PJ