Sending and Receiving Mail (Tbilisi)

How exactly does the postal system work here? Do I have to physically go to the post office or can I buy stamps to send a postcard back home? Also, I know my address, but there doesn't appear to be any sort of mailbox in the vicinity of my building- Can I get a p.o. box in Tbilisi? I also only know of one post office in the whole city. Are there more?