I have the same problem. Here is their Customer service number so you can cancel Prime and they can refund you the $99.00.
1-888-280-4331. I got this number not from Amazon but from my bank when I called to dispute the charge on my bank statement. What they did to you is outright fraud. Perhaps, a letter to the Better Business Bureau or the Federal Trade Commission's site at http://www.consumer.ftc.gov/articles/0162-alternative-dispute-resolution could help you recover your losses and change Amazon's policy of fleecing it's customers.
Mine was resolved. Amazon says they will refund me the whole $106.92 and the cancellation fee of $21.98 for the change I made today of a month-to-month charge of $10.79 which will be credited to my Amazon account. (The month-to-month of $10.79 was changed from $106.92 annual fee by me today trying to avoid the $21.98 cancellation fee. Husband said to call our bank instead and resolve it that way).
All this nonsense because I didn't notice I was being charged for the Prime service when I agreed to the 2-day free shipping for my purchase I made what back in May, 2015! This nonsense charge should be in HUGE letters on the check-out list when checking out your Amazon order and not in "fine print" to discover 2 months later on your bank statement. Your dispute, on the other hand, was done on the phone with an Amazon representative.
There is no excuse for what Amazon did to you. I hope this helps you recover all that you lost!
Product or Service Mentioned: Amazon Shipping Service.
Reason of review: Problems with payment.
Amazon Pros: Getting great deals, Quick service, Product selection.
Amazon Cons: Shipping policy.