Customers screenshot invoices. Auditors reprint them. Banks match them. If your system “updates” a sent document to include a usage line that arrived late, you have created two realities: the one in the customer’s inbox and the one in yours.
Sonic’s rule is dull and non-negotiable. Unpaid drafts and in-progress documents may still accept lines. Anything final, sent, void, or paid does not. Regeneration after a subscription gate resolves must still respect product-line identity so you do not duplicate what already billed.
Late usage belongs on a later invoice. A wrong quantity belongs on a credit note. An upgrade that changes the world going forward belongs on the schedule, not on archaeology of last Tuesday’s PDF.
This is slower than “just fix it.” It is how you remain able to answer, without sweating, what the customer was asked to pay.