I've recently became a very frequent online shopper and even though Amazon is sometimes cheaper, with the occasional online discounts, and the various Reward events (like 5-10X the Plum Points) I think it evens out. There's also a pretty decent selection of the books I'm interested in (mainly Manga, YA books, and some Comics).
They generally, especially in the past year, do a very good job with packaging. I also get my books on average within 2-4 business days of ordering them, and I always choose the "Free Shipping" option. It's definitely not perfect though, as I end up returning about 15% of my orders due to damage, but to be fair over 10% of that is usually small stuff (slightly tattered edges/spines as oppose to warping/bent pages) as I am quite anal about the condition of my books. Overall I'd have to say that the wide range of payment options and Reward opportunities is what keeps me coming back.