curl --request GET \
  --url https://{defaultHost}/v1/shipping_providers/{shipping_provider_id}/carrier_accounts \
  --header 'Authorization: Bearer <token>'

{
  "object": "list",
  "data": [
    {
      "object": "carrier_account",
      "carrier": "usps",
      "carrier_name": "USPS",
      "service_levels": [
        {
          "token": "usps_priority",
          "name": "Priority Mail"
        },
        {
          "token": "usps_priority_express",
          "name": "Priority Mail Express"
        }
      ]
    }
  ]
}