Get Upcoming Invoice

curl --request GET \
  --url https://api.monk.com/v1/customers/{customerId}/upcoming-invoice \
  --header 'Authorization: Bearer <token>'

{
  "data": {
    "id": "inv-a1b2c3d4",
    "invoiceNumber": "INV-00042",
    "contractId": "c1d2e3f4",
    "status": "Drafted",
    "invoiceDate": "2026-03-01",
    "dueDate": "2026-03-31",
    "totalCents": 15250,
    "lineItems": [
      {
        "id": "li-1234",
        "name": "Platform Fee",
        "quantity": 1,
        "unitPriceCents": 9900,
        "totalAmountCents": 9900,
        "priceId": "price-5678",
        "isMetered": false,
        "servicePeriod": {
          "start": "2026-03-01T00:00:00.000Z",
          "end": "2026-03-31T23:59:59.999Z"
        }
      },
      {
        "id": "li-5678",
        "name": "API Calls",
        "description": "5,350 units × $0.01/unit = $53.50",
        "quantity": 5350,
        "unitPriceCents": 1,
        "totalAmountCents": 5350,
        "priceId": "price-9012",
        "isMetered": true,
        "servicePeriod": {
          "start": "2026-03-01T00:00:00.000Z",
          "end": "2026-03-31T23:59:59.999Z"
        }
      }
    ]
  }
}