Despite what jerf said, which I whole heartedly agree with, there are plenty of interesting billing problems for a good engineer to sink their teeth into.

But there is also a long, long tail of boring problems that you won’t have time to do properly.

To put it a different way: even after you’ve solved the “mathematical” problems of billing, you still need to solve the operational stuff. Like payments integration, GL integration, invoice formatting, tax, reports, etc etc.

I think most engineers underestimate both the difficulty of the “mathematical” problems, and the length of the long, relatively boring tail.