Digital franking notes generated in customer systems and comprise cryptographic information that is used for verifying the postage indicia in local payment assurance systems. A data key used for generating the postage indicia is generated in a value transfer center and transmitted via a central payment assurance system to the local payment assurance systems, which import the key in order to verify franking notes generated using the key. The local payment assurance systems notify the value transfer center of successful key imports. After a successful import, the value transfer center issues a data key to the customer systems and postage indicia are verified by decrypting the contained cryptographic information in the local payment assurance systems.