To add bottle deposits or cup fees, add a modifier with one option with the price of a bottle deposit.

To make sure that the bottle deposit is not taxed, add the tax_category property with value NO_TAX on the bottle deposit modifier.

{
  "name": "Bottled Soda",
  "description": "",
  "plu": "5",
  "price": 300,
  "modifiers": [
    {
      "name": "Bottle Deposit",
      "description": "",
      "pick": { "exactly": 1, "pick_same_option": false },
      "options": [
        {
          "name": "Bottle Deposit",
          "description": "",
          "plu": "20",
          "price": 5,
          "modifiers": [],
          "includes": [],
          "ingredients": [],
          "tax_category": "NO_TAX"
        }
      ]
    }
  ],
  "includes": [],
  "ingredients": []
}