Pottermore itself heavily implies that the ebooks and audiobooks will all be available at once, despite the site having staggered content release.
Quote:
At launch, the site allows visitors to experience and explore the first Harry Potter story, with subsequent stories unveiled over time.
Also, in October, the Pottermore Shop will open, where readers can exclusively purchase and download the digital audio books and, for the first time, the eBooks, with the complete series available in a range of popular language editions - and more languages to be added later.
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As for the reader itself, I wouldn't be surprised if they end up selling HP covers and the like, but they may be waiting until the Pottermore Shop goes live to unveil them.