As of now, there have 2 entries snubbed out of Wacky Packages postcards series:
"Croca-Cola Zero" (originally planned for Wacky Packages postcards series 5 despite being only a promo sticker)
"Clubbed Crackers" (originally planned for the recent Wacky Packages postcards series
I wonder if there have been other rejected Wacky Packages candidates for the postcard series...