Problems with homemade billing systems

Now, on a business level I acknowledge that frequently the monetary costs of the billing system are negligible compared to the business advantages of the "correct" billing system. My complaint is more that the scribbled note often ignores what the rest of the company is doing for billing, e.g, "yes usage based billing is great but do you really need to bill on furlongs per fortnight when the rest of the company is billing based on seats or some basic, easy-to-understand resource allocation?", ignores that maybe there's an easy thing that's close to what you said but since it reuses all the existing stuff you can have it in two weeks instead of four months, etc. Do you really need to issue a discount coupon that takes off a percentage proportional to the length of the customer's first name, unless the customer is in a jurisdiction where that's illegal in which case we roll a die, unless the customer is in a jurisdiction where that's illegal in which case we give them $10 off their first $100 and take off every prime-numbered dollar after that? Because I've got code that just takes 10% off their first month right here. Do you really need to bill on the first of the month unconditionally when the rest of the company bills based on subscription start date, or vice versa? Even if you're not too worried about the costs of paying 6 months of developers, I bet you are worried about those 6 months of opportunity cost.

I can't even count the months of delays caused by treating napkin scribbles as carved stone I've personally witnessed, and I'm only tangentially involved in billing over all.

And somehow managers who are deeply familiar with costs and benefits and tradeoffs and make sensible decisions all the time just become the most obstinate customers when it comes to billing. No matter how small the tweak proposed and how many months forward it may bring your release date to do something just slightly different (and consistent with the rest of the company's existing policies), they will go to the wall for their billing deviation, even when it was frankly clearly nothing more than a whim or a transient thought at some point by somebody somewhere of no strategic or marketing consequence.

It isn't just engineers who underestimate the complexity of billing until it's too late; it's everyone, really. "Just" print an invoice turns out not to be so simple.