curl --request POST \
  --url https://api.withflex.com/v1/checkout/sessions/{id}/refund_allocation \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "checkout_session": {
    "line_items": []
  }
}
'

{
  "allocations": [
    {
      "payment_method_id": "obj_01EXAMPLE00000000000000000",
      "last4": "4242",
      "amount": 0
    }
  ]
}