Documents required for Business "M" visa

You need to provide your passport and its photocopy, the application form, a passport-style photo, an invitation letter of a duly authorized unit, an invitation letter from your business partner in China, local company covering letter, recent 3-month bank statement, and copy of business registration certificate.

My purpose is develop the new product and buy. Normally buying products are changed order by order. So the company desired to one person keep in china within one year. so what is the suitable duly authorized unit in below unit for getting a invitation later.

Municipal Foreign Affair Office
Municipal Bureau of Commerce
Foreign Economic Relations & Trade Commission
International Trade Promotion Commission
Foreign Trade and Economic Cooperation Bureau

Actually, you cannot apply for the invitation from a duly authorized unit on your own.