curl --request GET \
  --url https://app.flowglad.com/api/v1/customers/{externalId}/usage-balances \
  --header 'Authorization: <api-key>'

{
  "usageMeterBalances": [
    {
      "id": "<string>",
      "createdAt": 0,
      "updatedAt": 0,
      "livemode": true,
      "organizationId": "<string>",
      "name": "<string>",
      "pricingModelId": "<string>",
      "slug": "<string>",
      "aggregationType": "sum",
      "availableBalance": 123,
      "subscriptionId": "<string>"
    }
  ]
}