Software Payments

Payments & Billing Engineering

Use this skill to design, implement, and debug production payment integrations: checkout flows, subscription lifecycle handling, billing portals, entitlement gating, webhooks, regional pricing, and payment-testing strategy.

Keep the skill focused on system choice and durable integration defaults. Push provider-specific code detail, migration edge cases, and deep implementation examples into the references.

Quick Reference

When to Use This Skill

Use this skill when the primary work is:

• choosing a payment processor, merchant-of-record layer, or billing stack
• implementing hosted or custom checkout
• designing subscription lifecycle and webhook handling
• building feature gating and billing-portal flows
• handling regional pricing, tax-sensitive platform choice, or local methods
• testing and operating checkout or billing workflows

Route elsewhere when the main task is:

Defaults

• Stripe is the default processor for most SaaS and product teams.
• Hosted checkout is the default starting point unless branded custom UI is a hard requirement.
• Webhooks are the source of truth for subscription and entitlement state.
• Initialize provider clients lazily; do not fail builds on missing secrets at import time.
• Omit payment_method_types unless you intentionally restrict available methods.
• Make every webhook handler signature-verified and idempotent.
• Keep feature gating enforced in three places: registry, API boundary, and UI paywall.
• Treat platform availability, API versions, preview features, and tax behavior as volatile and verify before final advice.

Workflow

1. Classify the business model:
   • one-time purchase
   • subscription
   • usage-based billing
   • marketplace or multi-party flow
   • mobile IAP or hybrid web-plus-app billing
2. Choose the stack:
   • processor
   • merchant-of-record layer if needed
   • billing orchestrator if lifecycle complexity justifies it
3. Choose the checkout surface:
   • Checkout or Payment Links for speed
   • Payment Element for custom UX
4. Implement the operational core:
   • webhook verification
   • idempotent processing
   • billing portal or self-serve management
   • entitlement sync and feature gating
5. Add testing, observability, and incident handling before calling the integration production-ready.

ASCII Flow

Payments task
  -> Define money movement, provider, customer, and ledger boundary
  -> Classify charge, subscription, payout, refund, dispute, or wallet flow
  -> Design idempotency, webhooks, reconciliation, and audit trail
  -> Implement failure handling, retries, and customer-safe messaging
  -> Verify provider rules, network behavior, and compliance constraints
  -> Test success, decline, duplicate, timeout, refund, and retry paths

Platform Selection Defaults

Use references/platform-comparison.md and references/uk-eu-payments-guide.md for the detailed trade-offs.

Known Traps

• Hybrid mobile billing without one entitlement registry. If App Store, Play Billing, and web subscriptions can all grant access, define one canonical entitlement state and one conflict-resolution rule before launch.
• Treating checkout success redirects as purchase truth. Redirects are UX only; provisioning must wait for the verified webhook or store-server notification path.
• Mixing mobile digital-goods policy boundaries with web-SaaS billing assumptions. Apple and Google policy treatment for in-app digital goods is volatile and must be re-verified before final implementation advice.
• Assuming RevenueCat is automatically the default for every mobile subscription stack. It is a coordination layer, not a requirement, and adds its own operational surface.
• Reusing one webhook consumer for unrelated billing side effects without idempotent boundaries. Entitlements, invoicing, CRM sync, and analytics fan-out need separate retry and replay behavior.
• Deferring tax, merchant-of-record, and invoice-issuer decisions until after checkout buildout. Those choices affect product catalog, legal entity exposure, refunds, and support flows.

Common Anti-Patterns

• Selling iOS in-app digital goods through Stripe inside the native app flow.
• Creating separate product or price catalogs in web, mobile, and billing systems with no synchronization contract.
• Using provider object IDs as the only entitlement key instead of a stable internal subscription or account identity.
• Shipping a billing portal without a tested downgrade, cancellation, refund, and grace-period policy.
• Letting the pricing page, checkout copy, entitlement rules, and CRM lifecycle drift independently.
• Treating revenue analytics events as operational truth for access control.

Stripe Integration Defaults

If Stripe is the chosen processor, keep these defaults:

• initialize the server client lazily
• prefer Checkout or Payment Links first
• let Stripe choose dynamic payment methods unless business rules require restriction
• use webhooks, not the client redirect, as the source of truth
• store internal user and tier metadata carefully and validate it before DB use
• use subscription schedules for preplanned billing changes
• keep billing portal enabled for self-serve changes
• use entitlements when packaging changes often, but still enforce authorization in your own API

Use references/stripe-patterns.md for API-version notes, webhook mappings, and migration detail.

Production Readiness Checklist

• Webhook signatures verified on every handler before processing
• All webhook handlers are idempotent (safe to replay the same event)
• Checkout creation and webhook failures are explicitly logged
• Dead-letter or retry strategy defined for out-of-order or failing events
• Stripe CLI (or equivalent) wired into local and CI test flows
• Tests cover success, decline, and 3DS / step-up challenge flows
• Incident playbook written for checkout 500s, auth failures, and policy denials
• Tax and merchant-of-record decisions finalized before checkout buildout
• Feature gating enforced in three places: registry, API boundary, UI paywall

Common mistakes:

See: references/testing-patterns.md, references/webhook-reliability-patterns.md, references/feature-gating-patterns.md, references/ops-runbook-checkout-errors.md

Navigation

Core

• references/stripe-patterns.md
• references/platform-comparison.md
• references/subscription-lifecycle.md
• references/feature-gating-patterns.md

Regional and operational

• references/uk-eu-payments-guide.md
• references/regional-pricing-guide.md
• references/webhook-reliability-patterns.md
• references/ops-runbook-checkout-errors.md

Mobile and in-app purchase

• references/storekit2-native-patterns.md

Testing and migrations

• references/testing-patterns.md
• references/in-app-browser-checkout-contract.md
• assets/template-checkout-entrypoint-propagation-checklist.md
• data/sources.json

July 2026 Regulatory and Scheme Traps

• SCA 3DS2 test card matrix (Stripe): use 4000000000003220 to force a 3DS2 challenge flow; use 4000000000003063 for a frictionless 3DS2 authentication; use 4000002500003155 for a card requiring authentication; use 4000008400001629 for a card where 3DS2 is not supported (fallback to 3DS1 or no-auth). Verify current test card list in Stripe docs before CI test runs — card numbers and behaviour occasionally change between API versions.
• SCA exemptions apply to low-value and MIT transactions: merchant-initiated transactions (subscriptions, saved-payment charges) are exempt from SCA when correctly flagged with off_session: true and a valid mandate; failing to set this flag causes unexpected declines on renewal charges. EMV 3DS 2.3.1 (August 2022) remains the deployed spec; 2.3.1 is not a hard requirement for exemption handling, but issuer/network support for it varies — a v2.4 pilot with AI-based risk scoring and EU Digital Identity Wallet support is in early 2026 rollout and is not yet something to build against.
• PSD3 / PSR — agreed, not yet in force: the European Parliament and Council reached political agreement on PSD3 and the Payment Services Regulation (PSR) in November 2025; final texts were published in April 2026 and cleared committee/plenary votes in the following weeks. Official Journal publication was expected around mid-2026 but may slip later in the year — verify current status before citing an in-force date. The PSR is a Regulation (direct effect, no national transposition); PSD3 is a Directive requiring member-state transposition, typically 18–24 months after entry into force. Do not build to final PSD3/PSR requirements yet, but flag the expected changes below as architecture risks now.
• PSD3 / PSR changes to design around: mandatory open banking performance/API standards, revised SCA exemption thresholds, stronger consumer liability protections for APP fraud (aligning with the direction already set by UK PSR 2024 mandatory reimbursement), and expanded scope to one-leg-out (one side of the transaction outside the EEA) transactions.
• EU Instant Payments Regulation (IPR) milestones: eurozone PSPs have had to receive instant SEPA credit transfers since 9 January 2025 and send them since 9 October 2025, with Verification of Payee (VOP) required in the eurozone since October 2025. Non-euro EU member states have longer runways: VOP is not required until 9 July 2027, and the obligation to send instant credit transfers in euro from local-currency accounts can extend to 9 June 2028 under specified conditions. Re-verify exact dates against the regulation text before committing an implementation date to a customer.
• Visa Acquirer Monitoring Program (VAMP): Visa lowered the merchant "excessive" fraud-plus-dispute ratio threshold from 2.20% to 1.50% effective 1 April 2026 for the US, Canada, EU, and APAC (CEMEA remains at 2.20% as of this writing). The ratio merges fraud reports (TC40) and disputes (TC15) — a single disputed-and-flagged transaction can count twice. Acquirer-level thresholds (0.5% "above standard", 0.7% "excessive") also apply. Treat these exact percentages as volatile and verify against current Visa program documentation before advising a merchant near the line.
• Mastercard Excessive Chargeback Program (ECP): two tiers apply over a trailing two-month window — Excessive Chargeback Merchant (ECM) at roughly 100–299 chargebacks and a 1.5%–2.99% ratio, and High Excessive Chargeback Merchant (HECM) at 300+ chargebacks and a 3%+ ratio. Fines escalate from roughly $1,000 to $200,000+ at HECM, with possible MATCH listing. Verify current thresholds and fee schedules in the Mastercard rulebook before quoting numbers to a merchant.
• PCI DSS v4.0.1: this is the sole active version of the standard (v4.0 was retired; v4.0.1 is a clarification-only release, no new requirements). All 51 future-dated requirements from v4.0 became mandatory on 31 March 2025 and are now baseline, not aspirational — assessors expect them in scope (e.g., authenticated internal vulnerability scans, phishing-resistant MFA for CDE access, targeted risk analyses). Route full compliance scoping to a security/compliance specialist; this skill covers scope-reduction choices (SAQ A vs A-EP vs D) at the checkout-surface level.
• Stripe Managed Payments (merchant of record): Stripe announced general availability of Managed Payments in 2026 after a 2025 private beta, but rollout remains gradual and is gated by seller eligibility (initially concentrated on US-based sellers in good standing, digital-goods sellers, no Connect dependency). Verify current eligibility and supported surfaces directly with Stripe before committing to an MoR architecture built on it.
• UK BNPL regulation: the FCA's new regime for deferred payment credit (BNPL) enters into force 15 July 2026, requiring creditworthiness assessments before every agreement (including sub-£50 agreements, subject to a proportionate/outcomes-based approach), upfront disclosure, financial-difficulty support pathways, and FOS access. Firms could register for temporary permission from 15 May 2026 through 1 July 2026. This date is imminent relative to last-validated — re-verify it has actually taken effect before advising a BNPL integration as unregulated.
• UK PSD2 / FCA rules post-Brexit: UK SCA rules under FCA PSRs 2017 (as amended) remain in force and broadly mirror EU SCA with some divergences; the UK's mandatory APP-fraud reimbursement regime under PSR 2024 is already live and distinct from the EU's PSD3/PSR APP-fraud provisions still working through the legislative process — do not assume UK and EU consumer-liability rules are identical.
• Surcharging rules: surcharging on card payments remains prohibited for consumer cards in the EU and UK; verify local rules before any dynamic pricing or fee pass-through feature that touches card instruments.

Fact-Checking

• Known bugs, regressions, framework/compiler/runtime footguns, and version-specific crash or workaround guidance must be verified against current primary web sources before being treated as current fact.
• Verify provider availability, preview status, API-version behavior, and tax or compliance claims before final advice.
• Prefer official processor, merchant-of-record, and platform docs over secondary summaries.
• If live verification is unavailable, mark provider-specific claims as unverified.

Learnings Loop

Before applying this skill on a non-trivial task, read learnings.consolidated.md in this directory (and learnings.md if present).

After applying it, if you encountered a pattern worth remembering, a mistake worth preventing, or a domain fact that surprised you, append one dated bullet to learnings.md via agents-skills-feedback-loop/scripts/append_learning.py. Do not modify SKILL.md itself.