pay_pal.ex

defmodule PayPal.Transaction do
    @moduledoc """
    A Transaction structure. All *fields* are required:
      - description:  String. (Give a description to the transaction.)
      - *ammount*:
        - *currency*: String. (A 3-letter currency code. All supported
                              currencies are listed below.)
        - *total*:    String. (Total to be charged. 10 characters max with
                              support for 2 decimal places.)
        - details:
          - shipping:       String. (Amount charged for shipping. 10 characters
                                    max with support for 2 decimal places.)
          - subtotal:	      String.	(Amount of the subtotal of the items.
                                    Required if line items are specified. 10
                                    characters max, with support for 2 decimal
                                    places.)
          - tax: 	          String. (Amount charged for tax. 10 characters max
                                    with support for 2 decimal places.)

    ## Supported currencies     Currency Code
          Australian dollar     AUD
          Brazilian real** 	    BRL
          Canadian dollar       CAD
          Czech koruna          CZK
          Danish krone 	        DKK
          Euro 	                EUR
          Hong Kong dollar 	    HKD
          Hungarian forint* 	  HUF
          Israeli new shekel 	  ILS
          Japanese yen* 	      JPY
          Malaysian ringgit** 	MYR
          Mexican peso 	        MXN
          New Taiwan dollar*   	TWD
          New Zealand dollar 	  NZD
          Norwegian krone 	    NOK
          Philippine peso 	    PHP
          Polish złoty          PLN
          Pound sterling 	      GBP
          Russian ruble 	      RUB
          Singapore dollar 	    SGD
          Swedish krona 	      SEK
          Swiss franc 	        CHF
          Thai baht 	          THB
          Turkish lira** 	      TRY
          United States dollar 	USD

      * This currency does not support decimals. Passing a decimal amount will
        result in an error.
      ** This currency is supported as a payment currency and a currency balance
        for in-country PayPal accounts only.
    """
    @derive [Poison.Encoder]
    defstruct [description: "", ammount: [total: "", currency: "USD", details: [shipping: "", subtotal: "", tax: ""]]]
  end
  
  ## TRY THIS ON iex
  Poison.encode! %PayPal.Transaction{description: "yo"}

The error

(Poison.EncodeError) unable to encode value: {:details, [shipping: "", subtotal: "", tax: ""]}
    lib/poison/encoder.ex:339: Poison.Encoder.Any.encode/2
    lib/poison/encoder.ex:232: anonymous fn/3 in Poison.Encoder.List.encode/3
    lib/poison/encoder.ex:233: Poison.Encoder.List."-encode/3-lists^foldr/2-1-"/3
    lib/poison/encoder.ex:233: Poison.Encoder.List.encode/3
    lib/poison/encoder.ex:213: anonymous fn/4 in Poison.Encoder.Map.encode/3
    lib/poison/encoder.ex:214: Poison.Encoder.Map."-encode/3-lists^foldl/2-0-"/3
    lib/poison/encoder.ex:214: Poison.Encoder.Map.encode/3
    lib/poison.ex:41: Poison.encode!/2

Perhaps you need to do something like this:

defmodule PayPal.Transaction.Amount.Details do
  defstruct shipping: "", subtotal: "", tax: ""
end

defmodule PayPal.Transaction.Amount do
  defstruct total: "", currency: "USD", details: %PayPal.Transaction.Amount.Details{}
end

defmodule PayPal.Transaction do
  @derive [Poison.Encoder]
  defstruct description: "", amount: %PayPal.Transaction.Amount{}
end

Poison.encode! %PayPal.Transaction{description: "yo"}

Related issue in Poison: devinus/poison#32