As all heads are accounted for, replying to a wedding invitation is a must. So if you can't attend give a reason or if your generous give the bride and groom something to ease your absence.
Dear Sam,
Thank you so much for inviting me to your wedding in May. Unfortunately, I am unable to travel during that time. I'm certain you may understand why, I have not been fit for travel lately and I've been rather unwell.
Me and my wife Missy are however delighted that you have reach such a joyous and important decision in your lives. I know your celebration will be wonderful, my kisses to your bride Thia.
I'm enclosing a gift from your bridal registry and a small check for anything you might have plan for your honeymoon vacation, please accept it with my best wishes.
Love,
Uncle Marlowe