In China, the face value on the stamps is always valid,you can use it all around China.The only different is that the post office may change the tariff .For example:
The domestic letter tariff of less than 20 grams was mainly adjusted four times. The first time was in 1990, the tariff was adjusted from 0.04 yuan in the port and 0.08 yuan in the other port to 0.10 yuan in the port and 0.20 yuan in the other port. The second time was in 1996, the tariff was adjusted from 0.10 yuan in this port and 0.20 yuan in other ports to 0.50 yuan regardless of this port. The third time was in 1999, the tariff was adjusted from 0.50 yuan to 0.60 yuan in the local port and 0.80 yuan in the foreign port. The fourth time was in 2006, the tariff adjustment from the port 0.60 yuan, 0.80 yuan outside the port adjusted to the port 0.80 yuan, 1.20 yuan outside the port.
So,what you should do is just scrape together the expenses.