go to http://map.baidu.com/ and enter 邮局 into the search bar. look for the marker closest to you.
Sending packages from Post Offices is a bundle of fun if you can't speak Chinese - I did it in about 2004 and I doubt that things have improved. If you have a Chinese friend to help then they will probably know how to find the Post Office.
if it is a huge means more then 20 kg better go through a agent , you will get a better price then what you will get direct. but will take 2-3 days more then normal to deliver