I've done a from-scratch billing system build. As well as the complexities in the article, some of the logic required can result in rather exotic SQL.

Answering questions like "what was the maximum number of concurrent sessions per account between these two dates" with a SQL query is interesting. Making it perform properly adds a layer of fun.

This is just an ad for Stripe services.

In my experience the reason they are so painful is because stuff keeps getting tacked on and nobody wants to clean it up because they are afraid they are going to mess up some crazy business logic that a small fraction of customers depend on.

Basically business logic isn't logical.

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Throw in for B2B: large enterprises can actually negotiate their prices with vendors, so the “enterprise plan” might vary in price and structure.

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