Applying for a (Z) working visa in China

I was told by travelvisapro.com that to apply for a working visa in China i need to provide a business letter from a US employer stating why i will be going there, and how long i have worked for the specific company in the US, my title, ect... I am not going through any company. A friend gave my information to some schools in China and they have accepted me to come over and teach english for a year. I have my Invitation Letter of Duly Authorized Unit and my Work Permit. Is it still necessary to have a business letter from a US employer or will i be fine just submitting everything else that is required?