Two comments -
1) I'd extend the expiration date out a couple of years - personally I wouldn't want to buy a gag decal that will "expire" that soon, unless it was really cheap.
2) I'd offer an optional set of 'killed zombie' decals (like military pilots put on their planes to indicate their kills) - like a zombie stick figure. Maybe even make a variety of styles, upright zombie, crawling zombie, partially dismembered zombie, etc. You could also add a series of weapon silhouettes to go with each one, axe, chainsaw, shotgun, assault rifle, etc. - make them in contrasting colors and you can use them to "cross off" the zombies that were killed.
With the popularity of The Walking Dead, and the upcoming World War Z movie, I think zombie stuff is going to be a big seller for at least the next year - assuming the Mayans were wrong and we're not all wiped out in <30 days...