Nothing screams luxury more than a thick, stiff invitation. Conversely, invitations that are thin and bend easily send a message of poor quality and may reflect to the invitation recipient that the event will also be sub par. Thick cover stock is one way to achieve a luxurious feel. Another is to add a backing layer to the invitation.
A backing layer is a piece of cardstock adhered to the invitation itself. The backing layer is slightly larger than the invitation so it adds a border to the piece, creating an opportunity for an additional touch of color.
My favorite cardstock to use for a backing layer is Stardream metallic. Their 105 lb. stock can be purchased precut via cutcardstock.com or envelopemall.com, among others. I recommend the backing layer be sized .375″ greater than the invitation itself. For a 5″x7″ standard invitation for example, the backing layer would be 5″x7″ and the invitation would be 4.625″x 6.625″. This will achieve a .1875″ border all around the edge of the invitation.