The market is still tiny and while early adopters might be happy to offer their coin for nearly free for mixing, this might not be the case in the future. A 2% fee for achieving anonymity is still quite low, adding built in magic default values is not a very sustainable or helpful thing if you want to establish a free market.

Maybe display fee in relative and absolute values, optionally also with fiat prices?

Anyways, please put this warning value in a configuration file and maybe write your 2% as a default in there... but don't hide this deep in the code.

By the way, how high is traffic and volume actually? If I offer coins at relatively competitive rates, how often do these actually get taken? Daily? Once per hour? Are there historic stats to analyze?