When I read your comment, I thought the credits system sounded like it was going to be complicated or messy and involving usage. Turns out that they're just trying to make a potentially more affordable subscription model that automatically accounts for times when you're not using the software.

Each credit allows a month of use. Seems pretty straightforward, but I agree with you that because these aren't an auto-renewing subscription you could find yourself needing to pay at an unexpected time.