Warning... this deal is totally YMMV.
I went to the Fairfield, CA store today. And, Warhammer was "on sale" for $30. And, the NWN2 expansion was "on sale" for $25. They refused to let me use the coupon, even though I am a reward zone member, because of the fine print, that says not for use on sale items. I'm going to try again tomorrow, at some other locations, but i'm not pressing my luck.
They also informed me, that online prices, and store prices are different, because Best Buy online, is a different entity than Best Buy (just like Costco.com), and since it carries unique items, that are not available in stores, prices will tend to differ on the things that both of the stores (online, and offline) actually carry.
So, basically I could buy the game online for $20 and pick it up @ a store, but the coupon still wouldn't work, because it's an "in store only" coupon.