The motive and source of the attack remain unclear. CafePress.com said it was "working with the proper authorities" in trying to resolve and investigate the source of the attack. CafePress.com handles the website hosting, order fulfillment and payment processing on behalf of various store owners.
Their community forums are rife with news and discussion on the topic. With the vast number of compromised machines only increasing, attacks are often hard to trace, though in the past few years we have seen a few people arrested for being part of them.