That's right. Keep it very short and sweet, and include the information they need to process your request. "Dude, cancel me" probably isn't going to work so think it through and refer to the contract. Everything you need to cancel is in there.

If you have any equipment to return add that too, and tell them you will be returning it to the store (you might even mail it if it's just a card).

A properly written letter is really effective, and makes great evidence in court if things go wrong and the company fails to honor their side of the bargain. Been there, done that, and won.