Invitation Letter

We have booked a tourist trip to China, via an American company. I provided our flight details, and booking details together with the costs, the hotels, copies of our passports, our lichtna kartas - we live in Bulgaria. Our insurance details, the lot, even filled the form in in type which was requested, but obviously not adhered to in Bulgaria. All this not enough, they now want a personal invite from China for us to go. I have asked the American company to help, and so far no joy. Reading in other places it states this is not necessary if you have proved entry, departure, payment details etc. I am at a loss what to do, and due to travel mid May. Please any help. They would not listen at embassy.