Dealing with factory or distributors.

1. Must have an agreement which lists out all the terms & conditions.  Always go back to the agreement if the factory comes up with some ridiculous comments
2. Must have an agreement.
3. Speak the same language so you can negotiate better. 
4. If it's not that big of a deal, accept it and make it sounds like you are doing a big favor. 
5. Always have a backup plan. 
6. Be polite and courteous, yet firm in your request. 

I know these all sounds common sense, yet I hope I will always remember this and can be advisory for others who consider working with a third party factory.

Good luck!