I always use stiffened felt in my postcards and I find that it works well. It is also quite inexpensive compared to Peltex. I believe it would be available at most craft stores.
http://www.joann.com/stiffened-9inx1...prd821383.html
This is a link to a Joann store for a pack of 12 sheets....9 x 12 inches......$10.99. I am sure that you can also purchase by the sheet also at some stores.
I cut the felt just slightly smaller than the card and insert it before the back is sewn on. As I mostly machine embroider mine it is still very much the same process as Judith mentioned.