Answered by ZUPAN | Nov. 16, 2019 04:15
Ben, you have published your question too late. To be allowed to obtain an invitation letter you need to start your correspondence with Hainan based agency at least 3 or 4 days before your planned arrival.
I can just suggest you at the time being that you declare at check-in in your origin country that you will travel to Hainan by relying on Hainan visa on arrival (VOA). This is a legitimate way to reach Hainan and upon your arrival in Sanya, you will have to apply for this VOA and pay, as French citizens, a price of 168 CNY for the mentioned VOA.
Also, I hope that you are traveling from an airport where the airport staff will know at least something about Hainan VOA. I hope that your itinerary in order to reach Sanya is not through mainland China. OK, it could be through mainland China, but the last airport before Sanya has to be outside of mainland China. The best option is Hong Kong. So, if you feel that I could help you with some detail, just leave a message here.