Tax receipt. I was told by my agency that Hainan and Lucky Air dont provide that, but some guys working there can confirm that.
If you live in a country where you have to pay income taxes, this could be a deal breaker because even if your country has a tax agreement with China, you still need a tax receipt, otherwise you will have to pay for and now the high pay job it is not worthy anymore.
If you don't pay taxes at all, then never mind.