curl --request POST \
  --url https://api-eu.flexportal.io/v1/subscriptions/calculate-early-return-fee \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --header 'Tenant-ID: <tenant-id>' \
  --data '
{
  "rentalId": "<string>"
}
'

{
  "success": true,
  "earlyReturnFee": 123,
  "remainingMonths": 123,
  "penaltyPercentage": 123
}