What's the difference in meaning between stay at the post office and stay in the post office?
Stay in the post office = stay inside the walls of the post office. I would say this to someone who is already inside the post office, not in the parking lot. I would say this to someone who I want to stay in the same place, not moving, so I can find them easier.
"At" is generally used for a place.
Where is Mary?
She is at the post office.
She is at work.
She is at school.
She is at the office.
She is at church.
She is at the store. (usually means grocery store)
"In" is generally used for being 'contained' or 'surrounded', so to speak.
Where is Bob?
He is in his office. (a private office with walls)
He is in the living room.
He is in his bedroom.
He is in the bathroom.
He is in his car.
"On" is used with mean of public transportation.
Where is Mr. Black?
He is on a plane to Dallas.
He is on a train to Chicago.
He is on a bus.
These are challenging, I know, but I hope this is helpful to you.