curl --request PUT \
  --url https://api.upstackdata.com/accounts/api/products/{productId}/{variantId}/handling-fee \
  --header 'Content-Type: application/json' \
  --header 'x-api-key: <api-key>' \
  --header 'x-pixel-id: <api-key>' \
  --data '
{
  "handlingFee": 1
}
'

{
  "variant": {
    "id": "<string>",
    "productId": "<string>",
    "sku": "<string>",
    "title": "<string>",
    "handlingFee": 123,
    "manualCostHistory": [
      {
        "id": "<string>",
        "cost": 123,
        "effectiveStartDate": "<string>",
        "effectiveEndDate": "<string>",
        "description": "<string>"
      }
    ]
  }
}