Make an offer to the seller of a listing

{
  "type": "object",
  "properties": {
    "id": {
      "type": "string"
    },
    "message": {
      "type": "string",
      "description": "Message to include with counter offer"
    },
    "price": {
      "type": "string",
      "description": "Offer price"
    },
    "shipping_price": {
      "type": "string",
      "description": "Shipping price (sellers only)"
    }
  },
  "required": [
    "id",
    "price"
  ],
  "additionalProperties": false
}