Z Visa Application Process in Thailand

I am an American applying for a Chinese Z visa from Thailand. After I have received the Foreign Experts' Certificate and invitation letter from my employer, how do I go about applying for it? I know in the US, you cannot just mail in the application and documents -- if unable to go in person, one has to hire a visa agency to help. Is it the same in Thailand? If so, what are some visa agency options for me?

Thank you!