A normal post office. The staff are friendly and helpful. Since the department is located in the center, there are often queues. But here you can buy postcards for 0.3 euros, which in all tourist shops start from 0.8 euros.
On my first trip to Montenegro, it was here that I paid the tourist fee and registered.
It took 5 minutes. The staff did everything for me, all I had to do was provide the address, the length of stay and the money to pay.