build // Lightning · Wallets

Lightning Retail POS

Cashier flow: cart total → invoice QR → settle/expire. Idempotent tickets so refreshes never double-charge.

Advanced2 weeks

Overview

A Lightning POS is the canonical merchant integration: show amount → display invoice/QR → detect payment → mark order paid.

Key orders by ID and payment hash. Replaying webhooks must not double-fulfill. Prefer clarity and speed over ornamental UI - cashiers need huge type and an obvious paid state.

Learning objectives

Create BOLT11 (or LNURL-pay) invoices for cart totals
Subscribe to settlement and update order state safely
Deliver a cashier-friendly UX with expiry and failure handling

Prerequisites

Watchtowers
Invoices
Failure Handling

Required skills

BOLT11LN node APIsIdempotent webhooks

UX states

Part of the lightning roadmap.

Architecture diagram

Cashier UI ──► POS API ──► Lightning node
                 │
                 └──► order store + webhooks

Requirements

Create ticket / cart total in sats
Display QR + copyable BOLT11 (or LNURL-pay flow)
State machine: unpaid → pending → paid / expired
Huge amount + countdown expiry in the UI
Idempotent order fulfillment keyed by payment hash

Stretch goals

Tip-adjusted amounts without confusing duplicate tickets
Audio/visual paid confirmation for noisy shop floors
Offline-friendly retry when invoice subscriptions drop
Testnet/mainnet network label always visible

Recommended resources

Example repositories

BTCPay Server Greenfield
Merchant invoice APIs with Lightning support.
Alby
Wallet UX and LNURL patterns useful for POS flows.

Submission instructions

Repo + README covering node auth (macaroon/TLS or LNURL)
Demo video of create → scan/pay → paid confirmation
Tests for expiry and duplicate webhook delivery
Screenshots of unpaid, paid, and expired states
Lab evidence pack: Polar/testnode payment hash, expiry failure, and duplicate-webhook test output

Discussion

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