Hello everyone!!
I am planning to get a commercial support.
https://ardupilot.org/planner/docs/common-commercial-support.html
My questions are
How to pay for a particular company? and
How can I get to know the price for each of the company listed in that page?
Please let me know.
Thanks.
What did you find out? 
I have the same need.
You would email selected providers outlining your requirements and see what they say.
If you need physical onsite help then try contacting the companies in your region, or see if they know of someone in your region.
It’s not a fixed price list kind of thing.