code: def refund(transaction_id, caller_reference, options = {})
        submit Refund.new(options.merge(:transaction_id => transaction_id, :caller_reference => caller_reference))
      end
docs: Refunds a transaction. By default it will refund the entire transaction, but you can
   provide an amount for partial refunds.
  
   Sign up for Instant Payment Notifications to be told when this has finished.
  
   See http://docs.amazonwebservices.com/AmazonFPS/latest/FPSAdvancedGuide/Refund.html