The 20% off was a mistake as I am a Border's reward member and had the same coupon. It excludes the reader, so it was fortunate to get that deal. As for the $50 credit, the Borders staff where I bought mine didn't know about it either (although it is in their Ads and has been for weeks). When you purchase it, the register automatically prints out how to redeem the credit with your receipt, so you get that credit along with the $10 credit on the box.
So far I am very pleased with my Sony Reader as well, although the Borders/Sony Ebook Store leaves a lot to be desired. I could have put together something more professional and I'm just a hobbyist with programming and websites.