curl --request POST \
  --url https://api.example.com/v1/billing/invoices/calculate/ \
  --header 'Content-Type: application/json' \
  --data '
{
  "items": [
    {
      "type": "staff",
      "count": 123,
      "id": 123,
      "integration_period": 123
    }
  ]
}
'

{
  "items": [
    {
      "type": "staff",
      "count": 123,
      "price": "<string>",
      "id": 123,
      "integration_period": 123
    }
  ]
}