1. A unique amount per checkout. A 15 USDC product becomes 15.000437 USDC. The 1–999 micro-unit suffix is the order ID, so there are no accounts, no memo fields and no per-order wallets.

2. Watch the chain with eth_getLogs. Poll the USDC contract on Base for Transfer(from, to=shop, value) logs every few seconds, using a couple of public RPCs as fallbacks (some don't serve getLogs). When a log value matches a pending session, mark it paid.

3. Tx-hash fallback. If the poller misses it, the buyer pastes the hash and the server checks the receipt directly.

4. HMAC download tokens. A paid session gets a 24h signed link. SQLite holds the sessions. Nothing else.

5. Discounts without double-dipping. A code's uses are counted only from paid sessions, server-side, and the discount is baked into the unique amount.

6. Optional emailed receipt. The buyer can enter an email. It's used once for the link and receipt, then nulled in the DB.

Gotchas: public RPC getLogs limits, 6-decimal USDC, race conditions on limited codes, and QR codes (EIP-681 ethereum:<token>@8453/transfer?address=…&uint256=…).

It's running the Lodestone shop (templates, Notion, spreadsheets, dev kits): https://jackiscool.com/shop/. LAUNCH10 takes 10% off for the first 3 orders if you want to watch it work.