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How to setup workflow for billing clients

Hi, I am building an app for managing podcast production- We allow our clients a certain amount of minutes each month depending on which plan they are subscribed to. I am storing this as an item in the database “current minutes”. I would like to create a workflow that charges the client for relevant overages if their “current minutes” are less than 0 on their billing date. It should bill at a rate of ($x.xx) * (amount of minutes over limit.) I’m having trouble determining how to structure this as I have a negative number to work with if they go over. Any ideas?