Letter of Invite Process Inquiry

I am an American who will be working for a multinational company for one year contract in Hangzhou, China. I just received an email from the company that my work permit has been approved by the local government in Hangzhou. After supplying a return email with information such as: my city of residence, fax number, visa application location in which I will apply for visa. I was wondering what that information is needed for? Is it for the letter of invite? Please can someone explain the letter of invitation process and how long it takes to get it approved? Also, do most companies use mail express service to send such sensitive documents? I appreciate all feedback and answers.

Best Regards,
Aaron