I worked for a B2B SaaS outfit which charged by month by seat. Some customers demanded usage-based charges and our sales and PM said yes, without checking in wi

I worked for a B2B SaaS outfit which charged by month by seat. Some customers demanded usage-based charges and our sales and PM said yes, without checking in with dev. As the guy who knew the app's logs best, it fell to me to code up and then maintain the monthly report to generate those customers' usage data and send it to billing. (The things we do for love.) Our app's log data wasn't complete and predictable enough to generate completely accurate usage data, so some assumptions were needed. (The problem was sometimes-missing timestamps for "usage session complete". Yeah I know, WTF? I inherited that code.) Things got a tiny bit ugly. Customers -- whales of course -- started demanding weekly, then daily, usage reports. They demanded backing data for their bills. And, it became really hard to actually fix the defects in the app's logging so it was possible to generate accurate usage -- it would change the billing too much, and we'd have to explain it to the customer. It was a pain in the you-know-what. Especially for a guy like me who prefers to write really accurate software where other peoples' money is involved. This didn't threaten the business, it wasn't nearly that serious. But, wowee, it was unpleasant. Lesson: If you're going to add a new SaaS pricing model to an existing service, think of it as a BIG change. Make sure you can capture accurate and commercially defensible metrics for your pricing model. Get devs to think it through. Get testfolk to think it through and work out some system tests. Sales people, don't just tell customers they can have their own pricing model, but only if they sign up by the end of the quarter.