code: def reserve(value, currency, sender_token_id, caller_reference, options = {})
        submit Reserve.new(options.merge(
          :transaction_amount => {:value => value, :currency_code => currency},
          :sender_token_id => sender_token_id,
          :caller_reference => caller_reference
        ))
      end
docs: Begins a Reserve request for the sender token. Very similar to <tt>pay</tt>.
  
   Sign up for Instant Payment Notifications to be told when this has finished.
  
   See http://docs.amazonwebservices.com/AmazonFPS/latest/FPSAdvancedGuide/Reserve.html