Stack Exchange currently only accepts credit cards. However, they are using the Shopify service, which allows multiple payment types, including paypal.
http://www.shopify.com/faq#customer-payment-methods
Stack Exchange will have to create a business paypal account and provide that information to shopify in order to start collecting payments via paypal. Note, however, that they may have better rates through their current credit card processor than they would through paypal, so there may be an accounting decision here to specifically avoid paypal.
While they used paypal it early on for careers subscriptions, they have since incorporated and have a different business legal and accounting structure. Their previous paypal accounts may not be usable for business.
shop.stackexchange.com
(and redirects there fromstore.stackexchange.com
)? Looks like we'll need to have a synonym indeed.