lib/amazon_flex_pay/api.rb
def pay(value, currency, sender_token_id, caller_reference, options = {})
      submit Pay.new(options.merge(
        :transaction_amount => {:value => value, :currency_code => currency},
        :sender_token_id => sender_token_id,
        :caller_reference => caller_reference
      ))
    end
def pay(value, currency, sender_token_id, caller_reference, options = {})
      submit Pay.new(options.merge(
        :transaction_amount => {:value => value, :currency_code => currency},
        :sender_token_id => sender_token_id,
        :caller_reference => caller_reference
      ))
    end
Begins a Pay request for a sender token. If you are not also the recipient (this is a three-party marketplace
 transaction) then you must also specify the recipient token.

 Sign up for Instant Payment Notifications to be told when this has finished.

 See http://docs.amazonwebservices.com/AmazonFPS/latest/FPSAdvancedGuide/Pay.html