Abstract: The present invention, generally speaking, provides a flexible mechanism for effecting a payment/unlock transaction for deliver-then-pay content distribution. Instead of interacting with a local client interface, purchase is effected by interacting with a commerce Web site. The content is unlocked by delivering to the client a certificate, which serves as proof of purchase. The certificate is rendered secure so that it cannot simply be replicated to gain additional unauthorized access. In a preferred embodiment, a local application (e.g., a stand-alone application or a browser plug-in) is present on the end-user's machine and is registered with the local operating system and browser to handle files of a particular type used for certificates. Downloading and processing of the certificate may therefore be done transparently, without user-intervention. Piracy is prevented by “individuation” of the certificate.