Melio

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You cannot build the payment half of Melio, and building the rest badly is dangerous. Read this before starting.

**Moving money on behalf of a business is a licensed activity everywhere.** More importantly, the specific risk here is invoice fraud: the single most common corporate theft is a forged email changing a supplier's bank details, and a homemade payments tool is a very effective way to lose a large amount of money quickly.

What you can build is accounts payable up to the payment, which removes most of the work and none of the safety.

STACK
- Node 20+ with Fastify, server-rendered HTML
- SQLite through better-sqlite3, WAL mode
- Caddy in front

THE DATA MODEL
- suppliers: id, name, tax_id, bank_details_encrypted, verified_at, verified_by
- bank_detail_changes: id, supplier_id, before_hash, after_hash, changed_by, verified_by, verified_at — **append-only, and the most important table in the system**
- bills: id, supplier_id, number, issued_at, due_at, amount_cents, currency, file_path, status
- approvals: id, bill_id, approver_id, decision, note, at
- payment_batches: id, created_at, file_path, reference, released_by, released_at
- Money in integer minor units, always

THE FRAUD CONTROLS, WHICH ARE THE POINT
- **A change to a supplier's bank details freezes payments to that supplier until a second person verifies it out of band** — by telephone, on a number already on file, not one in the email. Record who verified and when
- Two people for any payment above a threshold, and the person who entered a bill can never be the person who approves it
- Show the last four digits of the account beside every scheduled payment, so a change is visible at the moment of release
- Duplicate detection on supplier plus invoice number plus amount, blocking rather than warning

THE ORDINARY WORK
- Bills captured by forwarding an email or uploading a PDF; read the number, date and total with OCR and pre-fill
- Approval routing by amount and cost centre, every decision recorded
- A due-date calendar and a weekly list of what is about to be late
- Export a payment file in your bank's format, and mark the batch released with its reference

RECONCILIATION
- Import the bank statement and match on reference and amount; anything unmatched is listed rather than silently ignored
- **A bill is only paid when the statement says so**, not when the file was exported

WHAT MATTERS MOST
The out-of-band verification on bank detail changes. Everything else here is bookkeeping; that one control is the difference between a useful tool and an expensive incident.