Idempotency: Every request has a unique ID. Retrying with same ID returns same result. Prevents double-charging.
Two-phase approach: First authorize (hold funds), then capture (take funds). If capture fails, release authorization.
Reconciliation: Compare your records with payment processor daily. Find and fix discrepancies.
Ledger: Double-entry bookkeeping. Every transaction has debit and credit entries. Sum must be zero. Audit trail for everything.