If you are invited to a wedding, the bride and groom consider you to hold a valuable place in their life and want to honor you by including you in this important and personal event. You should consider the wedding invitation to be an outward show of respect and a tribute to you.
Depending on the closeness of your relationship with them, you should feel inclined to send a gift to the couple whether or not you are in attendance the day of the nuptials. A wedding gift is more than a symbolic way of saying “Congratulations.” It should be thoughtful, personal and a way for the couple to be reminded of the bond between you and them.
The 5th Edition of Emily Post Wedding Etiquette points out that; “Following long-established traditions, everyone who receives a wedding invitation should send a gift, whether they attend the wedding or not." Emily Post also addresses the exceptions to this rule: "...if you send invitations to casual acquaintances, business associates you don’t know well or people you haven’t seen in years and they do not attend the festivities, then gifts are not expected.”