On October 23, Microsoft announced that they were going to introduce self-service purchase options for Microsoft 365 users on November 19, 2019. Microsoft says this was "due to customer demand," but I don't know a single customer that has EVER asked for this. <snarkymode>It was likely cooked up by someone at Microsoft who thought this would be an excellent way to boost adoption and help make their revenue targets.</snarkymode>
After getting a ton of REAL feedback from customers, Microsoft decided to delay implementation of this change till January 2020 and, more importantly, give tenant administrators a way to turn it off ahead of time. While this opt-out model is welcome, I believe an opt-in model is more appropriate. Organizations should be in control of their corporate data by default.
Fellow MVP Michel de Rooij wrote an article for the Enow ESE Blog explaining how to block self-service purchases. It only takes a few minutes to read and implement. Do it now or spend a lot more time explaining why you didn't do it later.