This is the entry to the Prague 1's tax office on Stepanska street. A lot of people was sending them their quarterly VAT tax forms. I knew it because I saw the same address on their letters at the post office while I was sending ours :)
We have a really beautiful main post office at Prague (at Jindrisska street). I have an inside picture of it ... somewhere. I'll try to find it. Well, you can see it on Wikipedia - all four pictures are from Prague's main post office. It is forbidden to take pictures here :)
And, finally, an outside photo in one older post: "Prague's biggest post office"