There is no such thing as a "billing unit". The data this field
contained was also a confusing mixture of two types:
- For emails and letters, it was just "notifications_sent".
- For SMS, it was the "chargeable_units" (billable * multiplier).
This replaces the single, ambiguous "billing_units" field with
"chargeable_units" and "notifications_sent" in both usage APIs.
Once Admin is using them we can remove the old field.