It's common for travel management companies to charge extra when booking a supplier requires more work or generates less revenue. This is the basic idea behind American Express Global Business Travel's additional $10 per transaction fee for certain airlines, revealed here on Tuesday. Sources contacted mainly by email during the past 24 hours indicated that other TMCs have not taken such a program this far. GBT, they said, is charging a lot. Its program is broad, and formal. The TMC execs among them like it . . .