Layover on Flight from Australia to Los Angeles with China Southern Airlines

Hello,

My wife and I are travelling to the US from Australia in September this year.
Apparently altho we dont need a visa, we do need to apply for an letter of invitation..is this correct? If so, where do we apply for this please?

We will only be in Gangzou (spelling ?) for 16 hours but we would like to book into a hotel and might need to leave the airport.

Can you advise please.

Thankyou

Regards,

Paul