# Tax and VAT Collection

## Objective

Collect the right billing evidence and let a tax provider work out the rate.

Location and tax-ID capture at checkout, calculation delegated to the billing or tax provider, and evidence stored with the invoice.

## 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. Never hard-code rates or maintain your own rate table. Rates change on political timetables and yours will be wrong within a year. Send the address, customer status and product type to the provider and use what it returns.
2. Collect the billing address and, for business customers, a tax ID. Validate the tax ID where the provider offers validation, but do not block checkout when the validation service is merely unavailable.
3. Store the evidence used for each transaction alongside the invoice: address, tax ID, validation result, rate applied, and the provider's decision. That is what an audit asks for, and it must reflect the moment of sale.
4. Decide whether displayed prices are tax-inclusive or tax-exclusive per market, and show the tax line separately on the invoice either way.
5. Handle reverse charge and exemption as provider-returned outcomes with their own invoice wording, not as a zero rate with no explanation.

## 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

- A valid tax ID changes who owes the tax. An invalid or unverifiable one must not silently be treated as valid — decide the fallback and state it to the customer.
- A customer moving country mid-subscription changes the rate on the next invoice, not retroactively.
- Tax-inclusive display in one market and exclusive in another means the same plan shows different headline prices. That is correct; make the UI handle it.
- Proration and discounts are taxed too. The tax figure must come from the same provider call that produced the amount, not be added afterwards.
- Digital and physical products can be taxed differently at the same address. Send the product type; do not assume one rate per customer.
- Stored tax evidence has a legal retention period that usually outlives the customer's account. Deleting an account must not delete its invoices.

## 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

- [ ] No tax rate is hard-coded anywhere in the codebase.
- [ ] Billing address is collected before purchase and tax ID collection is available for business customers.
- [ ] Each invoice stores the address, tax ID, validation outcome and rate used at the time of sale.
- [ ] Reverse charge and exemption produce their own explicit invoice wording.
- [ ] Inclusive and exclusive pricing are configured per market and the tax line is always itemised.
- [ ] Invoices and their tax evidence survive account deletion for the required retention period.
- [ ] 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.