I sent postcards to Russia. The 13th window (the rightmost one), the sweet girl understood my clumsy Serbian and pasted and sent everything for me herself. And I'm not the only one, as it turns out. Very customer-oriented :)
The central post office. If someone tried to deliver a package to your home and you were not at home, the package will be taken here. They work fast. In the morning, there is no one at all to receive. There are queues in the main hall, but there are many windows open.