curl --request GET \
  --url https://app.pennylane.com/api/external/v2/billing_subscriptions/{id} \
  --header 'Authorization: Bearer <token>'

{
  "id": 42,
  "next_occurrence": "2023-01-01",
  "prev_occurrence": "2023-01-01",
  "stopped_at": "2023-08-30T10:08:08.146343Z",
  "start": "2023-01-01",
  "finish": "2023-12-31",
  "status": "draft",
  "mode": "awaiting_validation",
  "activated_at": "2023-08-30T10:08:08.146343Z",
  "payment_conditions": "upon_receipt",
  "payment_method": "offline",
  "email_settings": {
    "recipients": [
      "jsmith@example.com"
    ],
    "billing_email_template": {
      "id": 123,
      "label": "<string>"
    }
  },
  "recurring_rule": {
    "day_of_month": [
      123
    ],
    "week_start": 123,
    "day": [
      123
    ],
    "rule_type": "monthly",
    "interval": 123,
    "count": 12,
    "until": "<string>"
  },
  "customer": {
    "id": 42,
    "url": "https://app.pennylane.com/api/external/v2/customers/42"
  },
  "customer_invoice_data": {
    "label": "Demo label",
    "currency": "EUR",
    "amount": "230.32",
    "currency_amount": "230.32",
    "currency_amount_before_tax": "196.32",
    "exchange_rate": "1.0",
    "currency_tax": "34.0",
    "language": "fr_FR",
    "customer_invoice_template": {
      "id": 123
    },
    "discount": {
      "type": "absolute",
      "value": "25"
    },
    "pdf_invoice_free_text": "Thanks for paying this invoice",
    "pdf_invoice_subject": "Invoice subject",
    "pdf_description": "Invoice description",
    "special_mention": "Additional details",
    "invoice_line_sections": {
      "url": "https://app.pennylane.com/api/external/v2/billing_subscriptions/42/invoice_line_sections"
    },
    "invoice_lines": {
      "url": "https://app.pennylane.com/api/external/v2/billing_subscriptions/42/invoice_lines"
    }
  },
  "created_at": "2023-08-30T10:08:08.146343Z",
  "updated_at": "2023-08-30T10:08:08.146343Z"
}