I also ran into the same situation yesterday at a Barnes & Noble. I returned the magazine in the opened poly bag and explained how the bonus poster map was not included. They gave me a full refund with no problem.
As an interesting side note, the bookseller and I checked the other two issues on the magazine stand, and they were missing the poster maps as well.
Long story short: Take it back with the receipt and they should give you a refund.
