Monthly Billing — Invoices & Generation

Monthly billing issues a per-period invoice (Draft → Issued → Partially Paid → Paid → Overdue → Void → Written Off). Generate Invoices reports five outcomes: Created, Backfilled (a new customer's missing prior periods), Skipped-already-generated (idempotent), Skipped-future-joiners and per-customer Errors — the run continues past an error.

Invoice lines are typed: Package Charge, Add-on, Tax, Late Fee, Pro-Ration, Adjustment and Discount. Payments allocate FIFO (oldest invoice first), and overpayment automatically becomes a Customer Credit.