Dear _________________
The _________________ (product) I purchased from you on ___________ (date) turned out to be quite a disappointment. While it looked the same as the one I saw in your commercial, it did not perform in the same way.
The operation which the product was supposed to perform does not act in the same way as I thought.
Therefore, I am returning the _______________ (product) to you and ask that you issue me a full refund. I am not interested in receiving a replacement.