There are 120 post offices in Warsaw and thousands of mailboxes that can be found on the streets.
The stamp for an "economical" postcard or letter (i.e. up to 6 days for Europe) costs 2.10zl and it does not depend on the destination.
The stamp for an "proritaire" postcard (i.e. up to 3 days for Europe, up to 6 days for the rest of the world) costs 2.60zl for zone (pl.: Strefa) A (European countries, Russia and Israel), 2.80zl for zone B (Africa and United States), 3.00zl for zone C (South America and Asia) and 3.60zl for zone D (Australia, New Zeland).
Please visit http://www.poczta-polska.pl for more details (Polish version only)