lib/amazon_flex_pay/api.rb
def refund(transaction_id, caller_reference, options = {})
      submit Refund.new(options.merge(:transaction_id => transaction_id, :caller_reference => caller_reference))
    end
def refund(transaction_id, caller_reference, options = {})
      submit Refund.new(options.merge(:transaction_id => transaction_id, :caller_reference => caller_reference))
    end
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