curl --request GET \
  --url https://api.getlago.com/api/v1/subscriptions/{external_id}/fixed_charges \
  --header 'Authorization: Bearer <token>'

{
  "fixed_charges": [
    {
      "lago_id": "1a901a90-1a90-1a90-1a90-1a901a901a90",
      "lago_add_on_id": "1a901a90-1a90-1a90-1a90-1a901a901a90",
      "invoice_display_name": "Setup fee",
      "add_on_code": "setup_fee",
      "created_at": "2023-06-27T19:43:42Z",
      "code": "setup_fee",
      "charge_model": "standard",
      "pay_in_advance": false,
      "prorated": false,
      "properties": {
        "amount": "30",
        "graduated_ranges": [
          {
            "from_value": 0,
            "to_value": 10,
            "flat_amount": "10",
            "per_unit_amount": "0.5"
          }
        ],
        "volume_ranges": [
          {
            "from_value": 0,
            "to_value": 10,
            "flat_amount": "10",
            "per_unit_amount": "0.5"
          }
        ]
      },
      "units": 1,
      "lago_parent_id": "1a901a90-1a90-1a90-1a90-1a901a901a90",
      "taxes": [
        {
          "lago_id": "1a901a90-1a90-1a90-1a90-1a901a901a90",
          "name": "TVA",
          "code": "french_standard_vat",
          "rate": 20,
          "applied_to_organization": true,
          "created_at": "2023-07-06T14:35:58Z",
          "description": "French standard VAT"
        }
      ]
    }
  ],
  "meta": {
    "current_page": 2,
    "total_pages": 4,
    "total_count": 70,
    "next_page": 3,
    "prev_page": 1
  }
}