curl --request GET \
  --url https://api.cloud.flexprice.io/v1/customers/{id}/invoices/summary \
  --header 'x-api-key: <api-key>'

{
  "customer_id": "<string>",
  "default_currency": "<string>",
  "summaries": [
    {
      "currency": "<string>",
      "customer_id": "<string>",
      "overdue_invoice_count": 123,
      "total_invoice_count": 123,
      "total_overdue_amount": 123,
      "total_revenue_amount": 123,
      "total_unpaid_amount": 123,
      "unpaid_fixed_charges": 123,
      "unpaid_invoice_count": 123,
      "unpaid_usage_charges": 123
    }
  ]
}