This is a not a service that would be done exclusively at the bank ICly. Most merchants would readily agree to do that if that would keep the adventurers coming to their shop, but code-wise, it has to be done somewhere, with a specific command.
I don't think there should be any percentage fee for changing money. From a game design point of view, this is unnecessarily penalizing: basically, those who already have lots of money won't bother that much, while those who have a hard time coming up with money will be greatly bothered (and will have to haul lots of heavy lesser-value coins).
