I would not use fusible fleece for the middle layer, not stiff enough. I'd use Fast2Fuse or Timtex. Here are a couple ways to make them.
http://debrichardson.com/blog/?p=232
http://quilting.about.com/od/quiltpa...c_postcard.htm
http://www.squidoo.com/fabricpostcards
There is a 'non-machineable' fee at the PO, I believe it's 20¢, because they should be hand cancelled. Keep the embellishments low profile and non-snaggy for best results. Otherwise they go through the mail just fine. Have fun!