When you get a copy of the registrar's contract, read every word and try to imagine the worst possible scenario. Some time back when I was casting about for a registrar, one sent me a contract which stated the following for audit costs:
XXXXXX "reserves the right to increase charges during the certification period".
Another said: "...approximately 45 days prior to the anniversary date of certificate issuance, XXXXXX shall notify the client in writing of the annual costs to maintain the certificate."
These appear to me to be licenses to steal. It would seem prudent to get these things tied down in your initial contract.