Looking on the internet deeply has found these results:

In the post office is the most popular phrase on the web.

At the post office 17,730,000 results on the web	In the post office 114,900,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
If you had left him alone, it would be at the post office safe and sound. And it's not because I'm at the post office. First in line at the post office. I saw Harmon at the post office. And it's not because I'm at the post office. Worked at the post office for 12 years. They're friends of mine from my days back at the Post Office. Derek Clark was being trussed up and beaten senseless at the Post Office.	He's been in the post office but I don't know him. You knew all the time that I found the bullet in the post office. Like send me a post card in the post office, whatever. It appears I will remain here, in the Post Office, after all. A year ago I was over in Tucson, they still had my picture up in the post office. Perhaps put a notice in the post office, explaining that it was a man's wages and his family needs it. I thought I recognized Sprague from his picture in the post office. Ordinary postal savings certificates can be 'bought'in paper form any day in any Italian post office, by anyone who presents themselves in the post office with an identification document and with the amount in cash they wish to invest in the certificates.

At the post office

17,730,000 results on the web

In the post office

114,900,000 results on the web

More popular!

He's been in the post office but I don't know him.
You knew all the time that I found the bullet in the post office.
Like send me a post card in the post office, whatever.
It appears I will remain here, in the Post Office, after all.
A year ago I was over in Tucson, they still had my picture up in the post office.
Perhaps put a notice in the post office, explaining that it was a man's wages and his family needs it.
I thought I recognized Sprague from his picture in the post office.
Ordinary postal savings certificates can be 'bought'in paper form any day in any Italian post office, by anyone who presents themselves in the post office with an identification document and with the amount in cash they wish to invest in the certificates.