Here's where they stick people who sign up.
https://asknebula.com/app/terms
On two different pages, they explain:
6.4 Subscribers may also use the Auto refill feature to have their readings without interruption. This feature allows the topping-up of the Credit balance to automatically renew.
So, you supposedly have the option of turning auto refills on, which is off by default. But then they turn that right around on the very next term:
6.5 You expressly authorize the Company to automatically top-up your Credit balance whenever it falls below the minimum of 40 Credits, until you cancel, for the default amount of 150 credits that included in the Basic package for $19.99
Auto-refill off? No problem. They automatically top-up your credit balance with no input from you.