Kimberley sez, "XKCD is selling a number of the strips as signed prints for whatever amount you want to throw his way. Default is $15. He's calling it 'Radiohead style'".
Note on price: we get a lot of emails asking about donations. We like to send people something tangible for their money, so we're offering these prints Radiohead-style — you can choose what to pay for them (above the default $15). If you want to donate money to the site but don't want a bunch of merch, just order a print or two and set your own price. Thanks!