The present invention provides a microbilling system that integrates with existing billing systems and existing devices. In a preferred embodiment of the invention, the devices store digital certificates with short-lived expiration dates and specifying transaction limits, preferably using a trust management system. A provisioning agent periodically (e.g., once a day) issues the certificates over a secure channel to the devices. When conducting a purchase transaction, the devices exchange certificates via a communication channel (that need not be secure) and the purchasing device issues a digitally signed electronic check which is periodically deposited over an advantageously secure channel with a clearing service. The present invention enables microbilling transactions by embedding liability in the certificates.