Ok for the one-upmanship I raise you billing systems for telecommunications. 1000x the transactions per customer and 1000x the complexity of the business rules.
Eg. An SMS on an off peak time is cheaper. If the user makes 100 or more calls a month a special rate. How do you charge data per plan? If the user roams to another country there's an entirely new set of complex billing rules.
Eg. An SMS on an off peak time is cheaper. If the user makes 100 or more calls a month a special rate. How do you charge data per plan? If the user roams to another country there's an entirely new set of complex billing rules.