The wording to politely decline an invitation should use a form of thanks, as well as a reason for the declining of the invitation. For example, one could write "I thank you very much for the invitation to John and Mary's wedding. Unfortunately, I will be out of town visiting family on that weekend."
A polite letter of decline should certainly include more than two sentences, and could go on to ask how the celebrants are doing. The person who is declining should also wish the celebrants the best of luck, as well as to express deep regret for not being able to attend.