code: 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
docs: 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