Payment infrastructure for
cross-border merchants

# signed merchant request
X-Merchant-Id: M10023
X-Timestamp:   1755500400
X-Signature:   HMAC-SHA256(body, secret)

{
  "out_trade_no": "SO20260818-0007",
  "amount":       "1280.00",
  "currency":     "HKD",
  "country":      "HK",
  "method":       "fps",
  "notify_url":   "https://merchant.example/hook"
}

→ 200 routed provider_a · fee 1.20% · ledger #482913