Checkout: Accounting and integration

Specifies records, provider calls, webhooks, idempotency, reconciliation, taxes, and audit evidence for checkout. This reference fixes the adoption boundary, failure behavior, and observable evidence while leaving environment-specific limits configurable.

Package status: reference context ready for human review. The contract and test scenarios are complete, but no claim is made that an adopting implementation has passed them.

Decision

For Checkout, adopt an internal durable ledger reconciled with authenticated provider events. Implement the behavior through one authoritative write path, typed interfaces, explicit state, and idempotent side-effect handling. This package is a reference contract: the adopting team must choose numeric limits, providers, jurisdictions, and operational owners from evidence in its own environment.

Scope

• The Checkout actors, inputs, outputs, states, policy or schema versions, and externally visible outcomes described by this implementation block.
• Primary and alternate interfaces, background work, caches, integrations, support paths, and evidence that can exercise the same boundary.
• Adoption-specific configuration, rollout, recovery, and verification responsibilities needed to apply the reference safely.

Outside this block

• Choosing a universal vendor, framework, jurisdiction, numeric threshold, retention period, or service-level objective for every adopter.
• Claiming that packaged scenarios have run against a downstream implementation or that this reference grants legal, security, or accessibility certification.

Contract

• Represent money with an explicit currency and exact decimal or minor-unit value; never use binary floating point for ledger arithmetic.
• Give each externally visible financial intent a durable uniqueness key retained for the relevant ledger and audit lifetime.
• Authenticate provider callbacks, persist the raw event reference, and apply state transitions idempotently after current-state validation.
• Separate customer-facing, provider, entitlement, invoice, settlement, refund, and reconciliation states instead of treating one status as authoritative for all.
• Validate inputs once at the boundary and preserve the normalized representation used by downstream logic.
• Persist the business transition before or atomically with an outbox record; consumers deduplicate by durable operation identity.
• Create a server-authoritative checkout snapshot containing items, quantities, currency, prices, discounts, tax inputs, total, customer scope, and expiry before payment confirmation.
• Bind payment submission to one idempotency identity and reconcile ambiguous provider results before retrying, fulfilling, emailing, or allowing the cart to be charged again.

Implementation guidance

• Ship additive storage and interface changes first, backfill observably, then retire old paths after compatibility evidence.
• Model Checkout inputs, outputs, actors, states, invariants, side effects, and evidence before selecting framework or vendor details.
• Store the applicable Checkout contract or policy version with material state so migrations, replay, and support decisions remain attributable.
• Introduce the path behind controlled rollout, compare expected and observed outcomes, and keep a tested recovery route until adoption evidence is complete.

Failure handling and safeguards

• An ambiguous provider result remains pending reconciliation; the system neither grants duplicate value nor silently discards a financial obligation.
• Invalid or contradictory Checkout state is rejected or quarantined; the implementation never guesses a value that broadens authority or duplicates an effect.
• Retries are bounded and use durable identity; after exhaustion, work reaches an inspectable terminal state with an accountable owner.

Verification and operations

• Inject dependency failure before and after persistence and prove that retry converges on one externally visible outcome.
• Monitor Checkout success, denial, validation failure, dependency failure, retry exhaustion, and recovery by contract version without sensitive payload dimensions.
• Re-run paired positive and negative fixtures after policy, schema, dependency, migration, or boundary changes and preserve the resulting evidence.

Adoption assumptions

• Names and values in the example are a concrete fixture, not universal defaults; adopters replace them through documented evidence and ownership.
• The adopting system can provide authenticated identity, durable operation or record versions, bounded telemetry, and a controlled path for change.

The executable-looking examples in this package are fixtures and acceptance contracts. Run the collection validator to check structure and metadata, then translate and execute the scenarios in the target repository before recording implementation evidence.

References

• PCI Security Standards Council, PCI DSS (applies as of 2026-09-08)