PayPal/Venmo checkout flow — SDK buttons and wallet transaction support

[aron23]

Context

Project: PayPal/Venmo Integration
Priority: P0 — buyer-facing payment flow

Goal

Add PayPal and Venmo as payment methods in the SDK, following the existing wallet payment pattern (Apple Pay, Google Pay). Buyer clicks PayPal/Venmo button → popup approval → capture → transactedObserver callback.

Architecture

• PayPal JS SDK v6 loads inside secure-tags-lib messenger iframe (PCI isolation)
• New button web components render PayPal/Venmo branded buttons
• Eligibility check via sdkInstance.findEligibleMethods() determines button visibility
• Venmo eligibility: US buyer + USD + merchant has Venmo enabled
• Transaction flows through existing processWalletTransaction() messenger path
• PayPal popup handles buyer auth; onApprove triggers capture via paypal-service

Repos Involved

• payment-components — new button components, wallet type constants, messenger events, exports
• secure-tags-lib — messenger iframe loads PayPal JS SDK v6, handles wallet auth, relays to WebSocket
• tags-secure-socket — extend wallet_transaction.py to route PAYPAL/VENMO to paypal-service
• paypal-service — order create, capture, client token endpoints (already exists)

Acceptance

• PayPal button appears when merchant has PayPal onboarded and enabled
• Venmo button appears when additionally eligible (US, USD, merchant toggle)
• Clicking button opens PayPal popup, buyer approves, payment captures
• transactedObserver fires with standard PayTheory transaction response
• Cancel and error flows handled (popup closed, instrument declined, etc.)
• Buttons do NOT appear if merchant hasn't completed PayPal onboarding
• Amount conversion (cents → PayPal dollar string) handled correctly
• Metadata passthrough (reference, invoiceId, accountCode) mapped to PayPal fields
• Repo-level spec docs in payment-components and secure-tags-lib