As a foreign customer, there's a bit of a learning curve. I suppose like most European post office…read moreis, you are required to take a ticket before entering. Even though the machines have an English option, some of the translation is slightly confusing.
I took a ticket for what I thought I needed, but then my number wasn't called for a couple minutes despite it being empty. I walked to several windows and despite being seen, was not really acknowledged either. I finally showed one of them my ticket, but she gave me instructions in Czech, prompting me to go back to the front and getting a new ticket, this time selecting the correct option. My number was then called almost instantly.
The woman who took care of me was pleasant enough. It was not crowded, and there seem to be a lot of booths.