No, not that kind of post office!
Instead, let's take a look at post offices at various world's fairs. Here's a special station set up at the 1962 Seattle World's Fair. Visitors could buy the new stamp issued for the fair, then get their mail canceled with a special "Space Needle Station" postmark. It's interesting to see the sign over the door reads "Space Needle, Washington" and not "Seattle, Washington." I'm sure most post offices are listed by the city they're in and not the building. In this case, the post office wasn't even in the Space Needle!