curl --request GET \
  --url https://api.steppay.kr/api/v1/subscriptions/{subscriptionId}/calculate \
  --header 'Secret-Token: <api-key>'

{
  "amount": 123,
  "items": [
    {
      "subscriptionItemId": 123,
      "amount": 123,
      "quantity": 123,
      "name": "<string>",
      "currency": "<string>",
      "count": 123,
      "unit": "<string>",
      "additionalBillings": {
        "id": 123,
        "ranges": [
          {
            "id": 123,
            "until": 123,
            "price": 123
          }
        ]
      },
      "usageFormula": "<string>"
    }
  ],
  "startDate": "2023-11-07T05:31:56Z",
  "endDate": "2023-11-07T05:31:56Z"
}