# Final Accounting Lock Rules

These rules protect money movements from accidental loss or duplication.

## Settlements

- A shipment can appear in only one active COD settlement.
- A paid settlement cannot be cancelled.
- A cancelled settlement must release linked shipments only if unpaid.
- Settlement approval must create a cash movement.

## Invoices

- One COD settlement should not generate multiple active invoices.
- Approved or paid invoices cannot be edited directly.
- Corrections must use cancellation or credit/reversal documents.

## Cash Ledger

- Cash movements are append-only.
- Never hard-delete cash movement records.
- Reversals must create opposite-direction entries.

## Shipment Financials

- COD changes after dispatch require a financial change log.
- Delivery fee, overweight, tax and COD fees must be calculated through `PricingEngine` only.
- Controllers must not contain independent financial formulas.