Do I have to go to the embassy personally to tramit a Visa or can it be done through mail?

I am a Mexican Citizen working in Krabi, Thailand. I will be finishing my work in June 13, travelling to Bangkok on June 14 morning, and then travelling to Shanghai on June 16, Morning. I would like to know if I can apply for my Chinese Visa through Mail, or if I have to go personally to the embassy. This is due to the fact that I will be only very short time in Bangkok before leaving to Shanghai. If there is any way of doing this through the mail it would be really helpful. If I have to go I have read of a Visa Application that only takes about 4 hours. Also, Do I need an appointment to go to the embassy, or do I just show up? Thank you very much for all your help.