A billing system implementation survivor. The subject is complex and everyone underestimates it by a lot. And the article does not even touch on the horrors of usage based billing and “flexible” pricing with add-ons.
I can recommend a mind-numbingly boring book on the subject that will teach you about some of the issues and hopefully help you avoid some of the design pitfalls. Another piece of advice would be to learn as much about accounting and bookkeeping as possible.