Compare these two:
When you order directly from us, you get all the DRM free formats a title will come in, plus lifetime updates to formatting or errata. For titles that we publish, there are also discounts on the print versions and any new editions that may be released in a series.
When you purchase from Amazon, you're only getting the Kindle version of a book, and none of the other features. Also, depending on how Amazon contracts with a given publisher (in this case Wiley / Sybex), they may be able to offer them at lower prices.
I hope that information is useful to you!
Customer Service Rep,