What is the rate of mailing an international letter in China from Shanghai to Singapore?

Do you choose China Post or other express? If you choose China Post, you have to pay 5.5 RMB with your package less than 20g. Extra money has to be paid when your package is over 20g. You have to pay another 1.5 RMB with addtional 10g.
If you choose other companies, you have to call the company for help directly.