Do I need an invitation letter from customer? I am going to Sanya for three days to run a workshop.

Yes, an invitation letter from your customer is required. It should include the following information：
1. the invitee's information: name, sex and date of birth
2. visiting purpose, when you arrive, when you leave, where you will stay and who burden your expense during your stay
3. Inviter's information: name, address, phone number and signature on the invitation letter