It makes sense that if someone wanted to bill for usage when invoicing they would need to have usage data available - and if you’re doing it at invoicing time you are essentially “back billing” for usage.
In our case - and I would imagine others as well, we are not really billing from the 20th to the 20th - our billing is per calendar month and the invoice on the 20th is for the upcoming month.
If we are billing per month, but charging usage from invoice date to invoice date I think that is going to be way more confusing for customers to understand as you now have two different “billing cycles” for them to manage.
Usage billing is already a post-action item - it seems like doing it via a specific time period is more of a WISP policy than anything else. As a suggestion, putting a flag to calculate usage on a calendar month or by invoice date or by X day of the month might be a way to generate usage totals, and then when invoice is created it could simply look to see if there were any existing usage periods to bill?
Just a thought.