I'd go a little farther and say that if there are impractical scenarios to avoid the fee it still must be included. (Say, no fee if you pay the fare in Timbukt

I'd go a little farther and say that if there are impractical scenarios to avoid the fee it still must be included. (Say, no fee if you pay the fare in Timbuktu.) Advertise it based on the 95th percentile of what actually happens.