Necessary documents for X1 type at the Airport.

I was turned away by Hainan Airlines trying to fly into Chengdu, unable to get past the check in desk because I did not have my official invitation letter. I cannot find any information anywhere that this is a requirement, as I already needed that letter to obtain my visa in the first place. Will a copy of this letter going to be accepted by Chinese customs as I am going to have to apply residency once I get there?