For the invitation for the Black/White Damask shower, I created an oversize postcard and had them printed at Vistaprint. I love the size and the glossy front side really made the design shine.
While they ARE postcards, they were mailed in white envelopes (available also at Vistaprint). On the outside, I used a matching envelope wrap. Very easy to use. Just print out and attach with a glue stick. The invitation and envelope wrap design is available in my etsy store here.