In my persistent quest to highlight bad user interface design, let me count the ways in which your store is broken.
1) When I hit the Buy Now button on an e-book, the store takes me straight to checkout. It does not put the book in a shopping cart. It does not allow me to continue shopping for more books. It forces me into a separate transaction for each book I buy. It forces me to enter my gift card information multiple times. This is ANNOYING. Why do you even bother displaying the word "Cart" in your menu, if there is no way to add anything to a cart?
2) If I run out of funds on one gift card, I cannot use a second gift card to top up the amount to pay on a given transaction. So if I have two gift cards, one with $5 left on it and one with $4 left on it, I cannot use them both to pay for a $9 book.
3) Your Playbook app is even more broken. It won't let me even BUY books with my gift cards. I enter their numbers, they get verified, your app tells me to click on Buy Now, and then DOESN'T DISPLAY A BUY NOW BUTTON.
I have previously complained to customer reps, but these problems persist. Way to make me go hunting for an alternate e-book supplier.