Create Shipping Rate

curl --request POST \
  --url https://api.withflex.com/v1/shipping_rates \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '{
  "shipping_rate": {
    "display_name": "<string>",
    "amount": 123,
    "metadata": {}
  }
}'

{
  "shipping_rate": {
    "shipping_rate_id": "<string>",
    "active": true,
    "display_name": "<string>",
    "amount": 123,
    "metadata": {},
    "created_at": "<string>"
  }
}