The calculation chimney is not component of the kesa service and must be separately ordered and invoiced. There is a performance specification which you can download here.
The kesa service is invoiced twice a year (in spring and in autumn). So you receive an invoice twice a year for each 6 months of kesa service.
When ordering you must pay the kesa service for at least one year. After that period, you can cancel your order for the kesa service in writing (terminate). If you cancel the kesa service possible outstanding amounts for program licences immediately become due.