# PayPal Checkout

## Objective

Accept PayPal payments with approval, capture, cancellation, and refund states tracked.

A server-driven PayPal payment flow that records each stage separately and reconciles the browser's return against the app's own record of what was paid.

## Before You Begin

This feature is being added to an application that already exists and already
works. Do not scaffold a new project, and do not assume a blank slate.

Inspect the codebase first and establish:

- The existing application structure and where code of this kind already lives.
- The framework and version in use.
- The existing design system — colours, spacing, typography, and component conventions.
- Existing UI components you can reuse instead of writing new ones.
- The existing database structure, if this feature needs to persist anything.
- The existing authentication and authorization system, if this feature is user-scoped.
- Dependencies already installed, so you don't add a library that duplicates one.
- The existing test setup and conventions.

Only start writing code once you understand the above. If the application
already implements part of this feature, extend it rather than replacing it.

## Implementation Instructions

1. Create the order on the server from trusted records. The browser may say which item is being bought and nothing about what it costs.
2. Model approval and capture as two distinct stages in the app's own data. A payer approving a payment is not the same as the money having moved, and collapsing them into one boolean is where this integration usually goes wrong.
3. Generate a stable reference for each attempt and use it on every capture request, so a retry after a timeout resolves to the original capture rather than taking the money a second time.
4. Verify incoming provider notifications as genuine and record their identifiers before acting, so retried deliveries update nothing twice.
5. If the app also carries a Stripe brief, keep one shared local record of a completed purchase and its entitlement, with each provider writing into it. Do not let two providers each maintain their own idea of whether the user has paid.

## UI and UX Requirements

Match the application's existing design system exactly. Reuse its components,
spacing, and typography. This feature should look like it was always there.

## Responsive Requirements

Works on mobile, tablet, and desktop. Touch targets are large enough to hit on a
phone, and nothing overflows horizontally at 320px.

## Accessibility Requirements

- Fully keyboard navigable.
- Correct semantic elements and ARIA roles.
- Visible focus states.
- Meets WCAG AA contrast.
- Dynamic changes are announced to screen readers.
- Respects prefers-reduced-motion.

## Edge Cases

- Prices, currency, and totals must be assembled server-side from records the user cannot edit. A total submitted by the browser is a suggestion from a stranger.
- An order can sit approved but uncaptured, or be captured for less than the full amount. Both need their own local state and their own operator-visible resolution, rather than being treated as either paid or not paid.
- Notifications from the provider must be signature-verified and deduplicated by their identifier, because they are retried until acknowledged.
- The payer's return to the app and the provider's server notification arrive independently and in either order. Resolve both against the app's own record of the attempt, and show a pending state rather than a contradiction while they disagree.
- A capture request that times out may still have succeeded. Never blindly retry the raw request; retry with the original reference or query the current state first, or you will double-charge.
- Refunds, partial refunds, and disputes change what the customer is entitled to. Decide what each does to access and reflect it in the same place the rest of the app reads.
- When the provider is unavailable, present the failure honestly and leave the order in a resumable state instead of clearing the cart.
- Currency support and payer country restrictions vary. Handle a rejection on those grounds with an explanation, not a generic error.

## Testing

Exercise the feature end to end in the running application. Cover every edge case
above, then run the existing test suite and confirm nothing regressed.

## Acceptance Criteria

- [ ] Orders are created server-side and no amount, currency, or quantity is accepted from the browser.
- [ ] Approved, captured, partially captured, cancelled, and refunded are distinct, visible local states.
- [ ] Provider notifications are signature-verified and processed once regardless of retries.
- [ ] The browser return and the server notification reconcile to one truthful state in either arrival order.
- [ ] A timed-out or repeated capture never results in a second charge.
- [ ] Refunds and partial refunds update entitlement through the same record the rest of the app reads.
- [ ] Provider outages leave the order resumable and the user informed.
- [ ] The feature matches the existing design system.
- [ ] No existing functionality is broken.

## Adaptation Rules

- Match the existing design system. Do not introduce a new colour palette,
  spacing scale, or component library.
- Reuse existing components and utilities wherever they fit.
- Follow the naming, file layout, and code style already present.
- Do not upgrade, replace, or remove existing dependencies to make this
  feature fit. Adapt the feature to the app, not the app to the feature.
- Do not break existing functionality. If a change is genuinely required in
  existing code, make the smallest one that works and say so.
- If something in these instructions conflicts with how the application is
  built, follow the application and explain the deviation.

## Final Verification

Before you report the work as done:

1. Re-read the acceptance criteria above and check each one against what you
   actually built.
2. Run the application and exercise the feature end to end.
3. Run the existing test suite and confirm you have broken nothing.
4. Check the feature on mobile, tablet, and desktop widths.
5. Check keyboard navigation and focus handling.
6. Summarize what changed: files added, files modified, and anything you
   deliberately did differently because of how this application is built.

If any acceptance criterion is unmet, fix it before reporting completion.