If all you need to do is send a postcard or a letter, you can buy stamps on pretty much any kiosk, just make sure they're right value for what you are sending and where. Once you put it on, drop your mail in any post box. These are the small yellow boxes attached to buildings around town.
Postal rates
Letters up to 50 gr Croatia 3.10 kn
Abroad 7.60 kn
Postcrads Croatia 2.80 kn
Abroad 4.60 kn