Beijing Post has launched a "future letter" service at nineteen locations. At the moment it is Beijing only.
You send a "future letter" with a delivery date of one year or several decades ahead.
There are no restrictions on delivery time and the cost of delifvery is charged by the year.
Beijing Morning Post (in Chinese) and Danwei (English) report that while the letter is still in storage the sender can ask to change the address and delivery time.