In article , Danny D.
wrote:

Is it illegal to snap a photograph of a clerk inside a post office?

Maybe. As a previous poster said, they have the right to ask you to
stop, and require you to leave if you don't. If you follow these
instructions, you've haven't broken any laws; if you don't, it's
trespass. It's important to know that USPS facilities are private
property. Despite all appearances, the post office is no longer part of
the government; legally, they're a private company that's been given a
monopoly contract to deliver the mail.

Regarding the postage exchange, that rule, as far as I know, changed
some time ago. A friendly counter clerk MAY change stamps for you, if
the stamps are usable and he can put them back in his inventory. Stuck
onto a piece of paper, no way.

I'm not a postal employee, but my SO is - this this is gleaned from
many, many "how was your day" conversations...