Sears review: interest charged unfairly

I have been a Sears customer for many years and have purchased all of my appliances as well as the warranty protection plan for them through Sears. This past February 2009, I was contacted by phone about renewing my warranty. I was told that the cost for 2 years would be $907.00 with 0 % interest. I am a senior citizen living on a fixed income so this was a major expense for me. I've always trusted Sears and have used the protection plan yearly to keep my appliances in good repair. My first bill arrived with no no interest charged, but the March bill arrived and I was charged 17.99% interest. I first called the credit department and was told that an error had been made on the computer, but it would be easier to take care of the problem if I contacted the warranty department which I did. They transferred me to someone at the national office who kept me on hold while they spoke to the warranty department and then told me to call the credit department . I told the young lady that I had already done that and suggested that she should speak to them first so they would have her imput. I held on and when she returned to the line, I was told that there was nothing that could be done to remedy the problem short of cancelling my warranty. This seems to be a terrible way to do business and I want a better answer to my problem. Actually I demand a better answer!