registrars management and pricing

How do I. a) Define prices (creation, update, EPP). b) Create credit for a zone-registrar combination. c) Deduce each EPP transaction (creation, updating, etc.) so the transaction applies to the credit?. I have scripts for creating a new registrar with their respective certificates, and I know how to intoroduce prices. Then I supposedly give credit to a registrar and a zone, but when I use that zone and registar using fred-client there is no transaction. I'm missing simething but what?. Mario Guerra