Don't you have a year to give a wedding gift? Or is it something I made up in my mind?
I agree with most everyone that you shouldn't really worry about this. A card letting them know that a special gift is on its way and will be sent to them as soon as it arrives should be enough. It is a polite way of letting them know that you didn't forget their special moment. I can't imagine the couple even realizing that your gift is not there. Besides, opening all the wedding gifts must be mind numbing and with all the fuss of opening the gifts, I think that special gifts kind of get lost in the shuffle. This way when all the excitement is calmed down, your gift will show up and they will be totally focused on your gift.
Your health is more important and you should safe guard it at all costs.