Handle underpayment and overpayment

1Branch on the webhook status

After verifying the HMAC, switch on status. underpaid and overpaid both carry the amount_usd actually received so you can compute the delta against the invoice price.

> { "invoice": "in_8fK2", "status": "underpaid", "amount_usd": 41 }

2Resolve an underpayment

For underpaid, keep the invoice open and ask the buyer to send the remaining balance to the same pay-to address. The next inbound transfer settles the invoice and flips status to settled.

3Resolve an overpayment

For overpaid, fulfil the order and return the excess with a partial refund against the invoice. A C2 write records the refund against the invoice id.

curl https://api.1st-node.com/v1/payments/invoices/in_8fK2/refund \
  -H "Authorization: Bearer sk_live_..." \
  -d '{"amount_usd":8}'

4Stay idempotent on retries

Webhook delivery is idempotent, so the same underpaid or overpaid event can arrive twice. Key your handler on the invoice id and status so a redelivery never issues a second refund or double-credits the order.