curl -X GET \
 /invoice-position-items/{id} \
 --header "Authorization: Bearer <token>"

{
  "id": {},
  "position": 123,
  "quantity": 1,
  "unit": {
    "id": {},
    "name": "Stück"
  },
  "unitPrice": {},
  "netAmount": {},
  "grossAmount": {},
  "taxAmount": {},
  "discountAmount": {},
  "discountPercentage": 10,
  "name": "My subscription product",
  "description": "* my subscription product",
  "taxGroup": {
    "id": "00000000-0000-0000-0000-000000000000",
    "internalDescription": "19%",
    "reverseChargeType": "REVERSE_CHARGE",
    "type": "standard"
  },
  "tax": {
    "id": "00000000-0000-0000-0000-000000000000",
    "description": "19%",
    "code": "19",
    "rate": 19
  },
  "type": "product",
  "parent": {},
  "group": {
    "id": "ad8a3b9e-5b0a-4e1a-9c1a-0b9b2b8b0b0b",
    "ranking": 1,
    "name": "Subscription #12345"
  },
  "createdAt": "2020-01-01T00:00:00+00:00",
  "product": {
    "id": "ad8f1c2c-3b1c-4b0a-8b0a-0b0b0b0b0b0b",
    "name": "Basic",
    "description": "For small teams.",
    "type": "product",
    "invoiceVisibility": "always",
    "customFields": {
      "field1": "value1",
      "field2": "value2"
    }
  },
  "subscriptionItem": {
    "id": "01F9ZQZJZJQZJZJZJZJZJZJZJZ",
    "name": "Fitness M",
    "description": "Everything included",
    "furtherInformation": "Additional agreements: The contract includes a free drink flatrate.",
    "status": "active",
    "subscriptionId": "<string>"
  },
  "serviceDateFrom": "2020-01-01T00:00:00+00:00",
  "serviceDateTo": "2020-01-01T00:00:00+00:00",
  "isCreditable": true,
  "isFullyCredited": true,
  "creditableQuantity": 1
}