They have something called a virtual credit card number. Your Paypal account would have a Mastercard # that you can use at any online store that accepts Mastercard (but generally wouldn't claim to accept Paypal), I think you get the # under the shops link. You cannot use it on a site that needs a credit card security code though since they don't give you one (I'm not sure if they do now or not since I haven't used it in over a year).