Billing systems are difficult to implement because of the complexity of billing rules that have been built up over the years.  Very often the municipality or utility struggles to describe its expectations in such a way that a vendor knows what is expected of them.  Once on site phrases such as “we thought it was obvious” are all too common.  A well-described User Requirements Specification or Checklist is an essential part of the selection process, and external consultants who have been through the process several times with multiple municipalities or utilities a key element.  Given the complexity of the billing rules, the consultants should themselves be experts in [1] public sector billing rather than generalist consultants or specialists in other types of software procurement such as ERP systems.