I appreciate your patience and have a quick update - you're seeing a monthly invoice because of a user overage. This means that one or more end users edited an app during the most recent service period, which puts them into the standard bucket.
This is explained in more detail in the Billing FAQ that is pinned here in the Community. I've included the relevant portion below.
I hope that explains why you received an invoice this month. In general, our Support team is the best point of contact for questions like this, but I'm happy to help. Let me know if you have any follow-up questions!