How can I get China business invitation letter?

I am a Nigerian living in Abuja. In the last 3 years I have been buying goods in 1*40 container from a Chinese company in Hangzhou. We do over 10 containers yearly. Recently, I asked them to get me an invitation letter to enable me visit them so as to put a face on the people I have been honestly dealing with in the last 3 years. To my chagrin, they said all their applications to invite me was turned down by the government on the premise that I have not visited china previously...They honestly advised that I should try other means, maybe pay an agent to sort me out...I really need candid advise from this forum as I need to explore the great opportunities abound in china...take my business to a new level. I have a great travel history...visited many European countries, America etc...