We have not allowed % option for per transaction below illustration will help you understand the aspect in a better perspective.
Just assume if we allowed you to keep per transaction surcharge and you kept the per transaction surcharge as 5.00% and the ticket price is 100 $ and customer is purchasing 3 tickets it would come 15 $ as a Surcharge amount. If you keep per ticket surcharge as 5.00% and if customer is buying one ticket at each transaction it would be $ 5 and for 3 ticket it would be $ 15. So it would remain same. That is the reason why we have not allowed the % option as a per transaction because it will not make any difference in terms of %. In per transaction surcharge you can decided a fixed amount and enter the amount accordingly.