### 1. Jira-Ready Epic & Core User Story Block * **Epic Link:** CHECKOUT-OPT-2026 * **Story Title:** Mobile Web 1-Click Express Checkout Bottom Sheet * **User Story:** As an authenticated returning mobile shopper, I want to confirm my order from the product page using my saved payment and delivery credentials, so that I can complete my purchase in seconds without navigating a multi-page checkout. * **Commercial Rationale:** Eliminates high mobile checkout abandonment rates by truncating a 5-step funnel into a single modal action. --- ### 2. Formal Gherkin Acceptance Criteria (BDD Suite) **Scenario 1: Successful Express Checkout (Happy Path)** Given an authenticated user is on an in-stock product display page And the user has a valid saved shipping address and active payment method on file When the user taps the Express Buy button Then the system displays the Express Checkout bottom sheet modal displaying default shipping and masked payment card ending in four digits When the user triggers the Confirm Purchase action Then the payment authorization request is dispatched to the payment gateway And the user is redirected to the Order Confirmation view within 1.5 seconds. **Scenario 2: Payment Token Expired or Card Declined** Given the user is on the Express Checkout bottom sheet modal When the user triggers the Confirm Purchase action And the payment gateway returns a token invalid or card expired error Then the bottom sheet remains open And the system displays an inline alert: Payment method unavailable, please select or add a card And provides a direct link to the payment management screen. **Scenario 3: 3D Secure SCA Challenge Required** Given the payment gateway requires strong customer authentication When the user confirms the purchase Then the system renders an in-app 3DS authentication modal And proceeds to order creation only upon verified authorization callback. --- ### 3. Edge Case Matrix & Negative Testing Boundary | Condition / Trigger | Edge Scenario | System Expected Behavior | | :--- | :--- | :--- | | **Inventory Depletion** | Item sells out while user has modal open. | Intercept charge request, close modal, display out-of-stock toast, refresh PDP state. | | **Network Interruption** | Connection drops during charge dispatch. | Implement idempotency key to prevent double charging; display retry prompt. | | **Session Invalidation** | Auth cookie expires while viewing modal. | Retain cart state, prompt biometric or PIN re-authentication, resume checkout. | --- ### 4. Technical Architecture Dependencies & Non-Functional Requirements * **API & Idempotency:** The POST /api/v2/checkout/express payload must pass an UUIDv4 idempotency key in the request header. * **Latency SLA:** Modal render time must not exceed 250ms on 4G networks; payment gateway response timeout capped at 4000ms. * **Telemetry Events:** Fire checkout_express_modal_opened, checkout_express_submitted, and checkout_express_completed.