It will depend on your state's insurance laws if your insurance company has the right to raise your premium after accepting your down payment. It is likely that after accepting your payment the insurance company finished its researching of your driving history, etc and found something that bumped up your premium according to their rating rules. This can happen, especially when you are changing companies and they are still gathering your personal information even after giving you a quote.

If you live in Florida and this occurs you have the right to pay the increase, request cancellation within 10 days or do not pay and the company will cancel you for nonpayment. The last choice is typically not a good one to make because it will show up on later when you are trying to find insurance with another company and be a negative for you.

To find out what laws your state has regarding the raising of rates in a situation like yours contact your state's insurance regulator.