I'm doing a job as an account officer in an NGO.
ACTULY MY ngO HAS A CONTRACT WITH a car rental company.this company send us a bill at higher rates but did not give us any notice,I want to ask from that company how they can increase rates without notice.how I write an email to that company.
Thanks

I assume that you just want to know what to say.
You don't ask how they can do it, you tell them that you aren't going to pay the higher rates, that you are witholding payments until this is resolved, and you go to the person in your company who does the car rentals and you notify him to stop until you find another rental company.
You do all this as fast as possible.
Chances are that if you are firm enough they will back down - but it's still a chance. Better start looking around.

Oh, and of course you read the fine print of the contract to make sure there isn't some sort of escalation clause in it first. I just assumed that you had done that.