Pretty much the topic - if you like buying things digitally on your WiiU or 3DS, this is as if you got 20% off any and all titles you ever decide to buy that way.http://www.bestbuy.com/site/promo/save-on-nintendo-eshop-cards-115703
(if you want to order online)
I've heard this is in their ad for this week, but I don't know for sure. Can't find the original source.
NOTE: Make sure you get the right cards. I've seen stores that still have the DSi points cards in stock - those things are next to worthless (unless you have a DSi anyway).