Simply Self Storage and Penske Truck Rentals

I was quoted a yearly rate for my storage units, which I paid in full at the time I moved in. I compared their quote to several places in the area. Moved in and paid in full for the year on 8/06/2009. Left with a receipt saying I was paid until 8/31/2010. Today they call saying I'm late in paying 5/5/2010. I showed them the receipt, thinking they had some error. (maybe wrong customer). But NO, they decided my yearly rate was too low and changed it in their computer without telling me to 8 months. So the quote they gave me for a year was just a lie to get my business. This is unethical, and I can't believe they looked me in the face and said "yeah, I see your receipt but we rolled back some months."