Billing Applications

Hey I was wondering if anyone has worked directly with billing/financial applications.. I have an iterview coming up for a billing application engineer, and would like some advice to the inner workings or non obvious stuff, maybe like certain design patterns, or major obstacles, etc... Thanks!

I work in something remotely similar in insurance.

Think on a high level as far as design. Think of the concept of parties (people/groups involved in the transactions), bills, mathematical algorithms or classes that will be needed to perform routine calculations, etc. Think of it all on an object level and how those objects will relate to each other.

That's all an interviewer (in my experience) will want to ask you about. They won't ask "what will you name this class and what methods and properties will it have?" Just start off thinking at a high level.