Dear Nui,
Thank you very much for your effort & support to us.
I understand Chinese Embassy in Bangkok is still insist on to stamp by Beijing Municipal Commission of Commerce P.R.C.
But invitation letter in attached has been prepared by the company not the government.
I have used attached invitation letter in South Africa and in Turkey to get Chine visa and I got it.
Please ask the embassy again, I believe you may have miss information.
Regards,
Sami