You should talk to a lawyer before you do anything. It's obvious you don't know where to start in the legal issue.

You don't just due someone. CarMax isn't a Rons Auto Sales car dealership, they're big time. They have lawyers on payroll for people like you and claims like this.

How much was your bill?
Were you covered by the warranty at the time of repair?
What was their reason for not honoring the warranty?
Is/was your car modded or did you do anything to void the warranty?
Have you called the warranty company to ask why your repairs were not covered?

If you're 100% certain you should be covered by the warranty, then file a formal letter of complaint to CarMax. Explain to them that you got the warranty with the understanding that it would be covered in such a matter that everything would be paid for except for the deductable. Give them a date (10 calendar days is usually ideal and ample time for them to respond) before you pursue any further action(s).

If you bought your car through CarMax, and if there's no other specifications to handle any legal claims or complaints with any other department, then send it to CarMax. If it goes to another department, then they will forward it or make them aware of the situation.

Wait out the 10 days, if nothing happens, then take legal action. If this is as cut and dry as you say it is, then you'll have no problem. Your lawyer will send a draft with your demands, and from there they will either tell you to piss off, or they will pay it.

Nine times out of ten it will cost less to settle the claim out of court, than to go to court. People tend to take you more seriously when they get a letter/draft from a lawyer.

Don't go around trying to sue everybody, and make sure you know what you're doing before you mess it up for yourself.

Call a lawyer.