To send off packages just go to the Post Office in the Food Mile on campus. Just bring you stuff in bags and the service personnel will help you pack it in the suitable packages on site (for a small fee).
Peking University Post & Telecom Office, 中国北京市海淀区五四路， 邮政编码: 100871 (map), Phone: +86 10-62757445
Packages can be sent by airmail, SAL (Sea Air Land) or sea with China Postal Service. Calculate 10 days, 2-3 weeks or 2-3 months for the shipment respectively.
If you have too much stuff, you can alternatively ask for a cheap pickup service (“Mr. Wang Express”, mobile: 13811185610). Mr. Wang is China Post staff and will come to your house, pack the stuff and post it for you. The next day he will come again with the receipt to charge you. For many semesters he proved to be a very reliable help.