It is considered proper etiquette to send a letter of congratulations to newlywed couples, especially if the sender of the letter has been invited to the wedding. Letters of congratulations do not have to be long, and can generally fit comfortably in a greeting card.
A letter to a newlywed couple should contain congratulations on the wedding, best wishes for the future and an acknowledgement that the couple is meant for each other or a smart match. If the sender of the letter is attending the wedding, it is appropriate to enclose the letter with the gift. If the sender is not attending, the letter should be sent by post so that it arrives within a reasonable amount of time following the ceremony.