const options = {
  method: 'GET',
  headers: {'X-Spree-Seller-Id': '<x-spree-seller-id>', Authorization: 'Bearer <token>'}
};

fetch('http://{defaultHost}/api/v3/seller/delivery_methods/calculators', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

curl --request GET \
  --url http://{defaultHost}/api/v3/seller/delivery_methods/calculators \
  --header 'Authorization: Bearer <token>' \
  --header 'X-Spree-Seller-Id: <x-spree-seller-id>'

{
  "data": [
    {
      "type": "flat_percent_item_total",
      "name": "Flat Percent",
      "preference_schema": [
        {
          "key": "flat_percent",
          "type": "decimal",
          "default": 0
        }
      ]
    },
    {
      "type": "flat_rate",
      "name": "Flat rate",
      "preference_schema": [
        {
          "key": "amounts",
          "type": "hash",
          "default": {}
        },
        {
          "key": "amount",
          "type": "decimal",
          "default": 0
        },
        {
          "key": "currency",
          "type": "string",
          "default": "USD"
        }
      ]
    },
    {
      "type": "flexi_rate",
      "name": "Flexible Rate per package item",
      "preference_schema": [
        {
          "key": "first_item",
          "type": "decimal",
          "default": 0
        },
        {
          "key": "additional_item",
          "type": "decimal",
          "default": 0
        },
        {
          "key": "max_items",
          "type": "integer",
          "default": 0
        },
        {
          "key": "currency",
          "type": "string",
          "default": "USD"
        }
      ]
    },
    {
      "type": "per_item",
      "name": "Flat rate per package item",
      "preference_schema": [
        {
          "key": "amounts",
          "type": "hash",
          "default": {}
        },
        {
          "key": "amount",
          "type": "decimal",
          "default": 0
        },
        {
          "key": "currency",
          "type": "string",
          "default": "USD"
        }
      ]
    },
    {
      "type": "price_sack",
      "name": "Price sack",
      "preference_schema": [
        {
          "key": "minimal_amount",
          "type": "decimal",
          "default": 0
        },
        {
          "key": "normal_amount",
          "type": "decimal",
          "default": 0
        },
        {
          "key": "discount_amount",
          "type": "decimal",
          "default": 0
        },
        {
          "key": "currency",
          "type": "string",
          "default": "USD"
        }
      ]
    },
    {
      "type": "digital_delivery",
      "name": "Digital Delivery",
      "preference_schema": [
        {
          "key": "amounts",
          "type": "hash",
          "default": {}
        },
        {
          "key": "amount",
          "type": "decimal",
          "default": 0
        },
        {
          "key": "currency",
          "type": "string",
          "default": "USD"
        }
      ]
    }
  ]
}